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Natural experiment

A natural experiment is an observational research design in which an exogenous event, policy change, or other non-researcher-controlled factor generates quasi-random variation in treatment assignment, approximating the conditions of a randomized controlled trial to facilitate causal inference.^[1] These designs are particularly valuable in fields like economics, epidemiology, and public policy where randomized experiments are infeasible or unethical, allowing researchers to estimate treatment effects by exploiting naturally occurring "as-if" randomization.^[2] One seminal early example is physician John Snow's 1854 investigation of a cholera outbreak in London's Soho district, where he mapped cases clustered around a contaminated water pump and demonstrated transmission via tainted water by removing the pump handle, effectively creating a natural intervention that reduced incidence.^[3] Snow further employed a comparative approach akin to a natural experiment by analyzing mortality differences between households supplied by rival water companies drawing from polluted versus cleaner sources during the 1854 epidemic.^[4] In modern economics, natural experiments gained prominence through studies leveraging policy discontinuities, such as quarter-of-birth variations in compulsory schooling laws to identify returns to education, as pioneered in the work of Joshua Angrist and Alan Krueger.^[5] The methodological rigor of natural experiments was recognized with the 2021 Nobel Prize in Economics awarded to David Card, Joshua Angrist, and Guido Imbens for their contributions to analyzing labor market effects using such quasi-experimental approaches.^[6] Despite their strengths in providing credible causal estimates from real-world data, natural experiments face scrutiny over the credibility of exogeneity assumptions, as untreated confounders may persist if the "natural" shock does not fully mimic randomization, necessitating robust econometric techniques like difference-in-differences or instrumental variables for validation.^[7]

Definition and Characteristics

Core Principles

Natural experiments rely on exogenous variation in treatment exposure, where changes in the independent variable arise from external events, policies, or rules beyond the researcher's manipulation, approximating random assignment in randomized controlled trials. This variation enables causal inference by creating comparable treated and control groups, provided the shock affects outcomes primarily through the treatment channel and does not correlate with unobserved confounders. For example, a policy implemented at a specific date or geographic boundary can serve as such a shock, isolating its effects if pre-existing trends are parallel across groups.^[8]^[9] Central to their validity is the as-if randomization assumption: the natural event must render treatment assignment uncorrelated with potential outcomes or individual characteristics that influence them, akin to a lottery or natural disaster's arbitrary impact. Researchers achieve identification through strategies like difference-in-differences, which compare changes over time between affected and unaffected units, or instrumental variables, where the exogenous shock instruments for endogenous treatment choices. These methods demand empirical tests, such as placebo analyses or falsification checks, to verify that the variation credibly isolates causal effects rather than spurious correlations.^[10]^[11] Unlike purely observational studies, natural experiments prioritize design-based inference, emphasizing the specific quasi-random mechanism over generalizability to broader populations, as outlined in econometric frameworks that treat them as localized randomized trials. This approach underscores transparency in describing the shock's plausibility and bounding potential biases from violations like anticipation effects or spillovers. While powerful for real-world settings where manipulation is infeasible—such as economic shocks or public health interventions—their internal validity hinges on contextual knowledge to rule out alternative explanations, often requiring multiple robustness checks across datasets or specifications.^[12]^[1]

Distinction from Controlled Experiments

Natural experiments differ from controlled experiments primarily in the assignment and manipulation of treatments. In controlled experiments, such as randomized controlled trials (RCTs), researchers deliberately manipulate one or more independent variables and randomly assign participants to treatment and control groups to minimize selection bias and confounding factors.^[13] This randomization ensures that observed differences in outcomes can be causally attributed to the treatment, as groups are balanced on both observable and unobservable characteristics ex ante.^[14] By contrast, natural experiments exploit exogenous variations in treatment exposure that occur independently of researcher intervention, often due to natural events, policy changes, or institutional rules, where the "assignment" mechanism mimics randomness without direct control.^[2]^[15] The lack of researcher control in natural experiments introduces methodological challenges absent in controlled settings. Researchers cannot standardize conditions or ensure compliance, relying instead on quasi-experimental techniques like instrumental variables, difference-in-differences, or regression discontinuity to isolate causal effects, which demand strong assumptions such as the exogeneity of the instrument or parallel trends between groups.^[13] For instance, while RCTs permit direct testing of treatment effects in isolated environments, natural experiments analyze real-world data where confounders may persist if the natural shock fails to fully randomize exposure, potentially requiring robustness checks to validate identification.^[7] This distinction underscores that natural experiments prioritize external validity in contexts where RCTs are infeasible—such as large-scale policy evaluations or ethical constraints—but at the cost of internal validity compared to the gold standard of randomization.^[16] Despite these differences, both approaches seek causal inference, with natural experiments often framed as observational analogs to RCTs when treatment allocation approximates randomness. Economists like Joshua Angrist and Jörn-Steffen Pischke have advanced this view, treating natural variations (e.g., draft lotteries or policy thresholds) as "as-if random" to address endogeneity in non-experimental data, though such designs remain vulnerable to violations of identifying assumptions that RCTs avoid through deliberate balance.^[17] Empirical studies confirm that while controlled experiments excel in precision for small-scale interventions, natural experiments enable broader applicability in fields like economics and public health, where manipulating variables at scale is impractical or prohibited.^[9]

Historical Development

Early Instances in Epidemiology

One of the earliest documented natural experiments in epidemiology occurred during the 1854 cholera outbreak in London's Soho district, investigated by physician John Snow. Snow plotted cholera deaths on a map, identifying a concentration of cases around the Broad Street pump, with 578 deaths recorded in a 250-yard radius, far exceeding surrounding areas. By interviewing residents and comparing attack rates—estimated at over 50% among Broad Street water users versus negligible in non-users—he attributed transmission to fecal contamination of the pump's well from a nearby infected infant. This exploited natural variation in water source selection based on proximity and habit, enabling causal inference without randomization.^[3]^[18]^[19] On September 8, 1854, following Snow's recommendation, local authorities removed the pump handle, after which new cholera cases in the immediate vicinity plummeted, with only sporadic reports thereafter despite ongoing epidemic elsewhere in London. This temporal association bolstered Snow's waterborne hypothesis against the dominant miasma theory, which posited atmospheric spread. Although the intervention's effect is confounded by the epidemic's natural decline, the pre-intervention spatial clustering and exposure gradient provided quasi-experimental evidence of causation. Snow's analysis highlighted how exogenous shocks, like sewage leakage into the water supply, create identifiable variation for epidemiological inference.^[20]^[21] Snow supplemented the Broad Street findings with a comparative study of water suppliers during the same outbreak, contrasting districts served by the Southwark and Vauxhall Company (drawing contaminated Thames water downstream) against those of the Lambeth Company (relocated upstream). Mortality from cholera reached 400 per 10,000 households in Southwark-supplied areas versus 37 per 10,000 in Lambeth-supplied ones, a tenfold difference persisting after adjusting for population density and prior sanitation. This policy-induced variation in exposure source exemplified an early use of natural experimental design to isolate environmental causal factors in disease outbreaks.^[22]^[23] In economics, the adoption of natural experiments accelerated during the 1990s amid the "credibility revolution," a shift toward quasi-experimental designs that prioritized causal identification over mere correlation in observational data.^[24] This movement addressed longstanding critiques of endogeneity in structural models by exploiting exogenous variations—such as policy discontinuities or unexpected shocks—as sources of plausibly random assignment.^[25] Pioneering applications included David Card's 1990 study of the 1980 Mariel Boatlift, which used the sudden influx of 125,000 Cuban migrants to Miami as an exogenous labor supply shock to assess wage impacts on low-skilled workers, finding minimal negative effects.^[26] Similarly, Joshua Angrist and Alan Krueger's 1991 analysis leveraged U.S. compulsory schooling laws, with quarter-of-birth serving as an instrument for years of education, to estimate a 7-10% return to an additional year of schooling.^[24] The methodological toolkit expanded with techniques like difference-in-differences and regression discontinuity, formalized in works such as Angrist and Krueger's extensions and Card and Krueger's 1994 examination of New Jersey's 1992 minimum wage hike compared to Pennsylvania's unchanged policy, which suggested no employment loss for fast-food restaurants.^[27] By the early 2000s, these approaches had permeated labor, public, and development economics, with over 20% of top journal articles in empirical microeconomics relying on such designs by 2010, reflecting a broader demand for policy-relevant evidence amid skepticism toward purely theoretical models.^[24] The 2021 Nobel Prize in Economic Sciences, awarded to Card, Angrist, and Guido Imbens, underscored this evolution, crediting their contributions to methodological foundations that enabled robust causal inference from real-world data.^[27] In broader social sciences, including political science and sociology, natural experiments gained traction post-2000, building on economic precedents to analyze institutional and behavioral outcomes.^[28] Thad Dunning's 2012 framework highlighted designs like instrumental variables for studying civil conflict or electoral reforms, emphasizing threats to validity such as spillovers.^[10] Applications proliferated, such as using lottery-based school assignments in Chile (circa 2000s) to evaluate private versus public education effects, revealing modest achievement gains from vouchers.^[28] This rise paralleled growing data availability from administrative records and censuses, though adoption lagged economics due to fewer quantifiable exogenous events in qualitative fields.^[29] Overall, the approach enhanced empirical rigor across disciplines, fostering interdisciplinary bridges like historical natural experiments in economic history.^[30]

Methodological Framework

Exogenous Variation and Identification

In natural experiments, exogenous variation refers to fluctuations in the treatment or explanatory variable that arise from sources external to the outcome of interest and uncorrelated with unobserved confounders, thereby enabling causal identification by approximating random assignment.^[31] This variation contrasts with endogenous changes driven by individual choices or feedback loops, which introduce bias in observational data; for instance, policy reforms that differentially affect geographic or demographic groups due to historical or institutional quirks provide such exogeneity when the assignment mechanism is plausibly independent of potential outcomes. Identification occurs when this variation isolates the causal effect, often through strategies like instrumental variables (IV), where the instrument—such as quarter of birth influencing compulsory schooling years—affects treatment but not the outcome directly except via the treatment.^[8] The credibility of exogeneity rests on the researcher's ability to argue that the variation is "as good as random," typically assessed via falsification tests, such as checking for pre-treatment balance across treated and control groups or placebo outcomes unaffected by the treatment.^[31] In Angrist and Krueger's 1991 analysis of U.S. Census data from 1910–1930, seasonal birth quarter patterns generated exogenous variation in schooling exposure due to school entry age rules, yielding an IV estimate of the returns to education at 7–10% per year after addressing potential confounders like family background. Similarly, natural disasters or lotteries—events where exposure is orthogonal to individual traits—serve as instruments, but weak instruments (low correlation with treatment) can inflate standard errors, necessitating checks like first-stage F-statistics exceeding 10 for robustness.^[31] Challenges to identification include violations of the exclusion restriction, where the instrument influences outcomes through channels other than the treatment, or monotonicity assumptions in heterogeneous effects settings, as formalized in Imbens and Angrist's local average treatment effect (LATE) framework, which identifies effects for "compliers" rather than the full population.^[8] Empirical validation often involves bounding exercises or sensitivity analyses to omitted variable bias, emphasizing that while natural experiments enhance internal validity over cross-sectional correlations, claims of causality require transparent discussion of threats like spillovers or general equilibrium effects.^[12] Advances in this area, recognized in the 2021 Nobel Prize in Economics awarded to Card, Angrist, and Imbens, underscore the role of such variation in bridging observational data to experimental rigor across economics, epidemiology, and social sciences.^[8]

Common Analytical Techniques

Analysts of natural experiments employ quasi-experimental designs to isolate causal effects from exogenous variations, such as policy shifts or environmental shocks, by comparing outcomes across affected and unaffected units while controlling for confounding factors.^[28] Key methods include difference-in-differences (DiD), regression discontinuity design (RDD), instrumental variables (IV), and interrupted time series (ITS), each leveraging specific features of the natural variation to approximate randomized assignment.^[1] These techniques prioritize identification strategies that rest on testable assumptions, such as parallel trends in DiD or continuity at thresholds in RDD, enabling robust inference without full experimental control.^[26] Difference-in-Differences (DiD) exploits temporal and cross-sectional variation by estimating the interaction between treatment status and a post-intervention indicator, assuming that in the absence of treatment, outcomes for treated and control groups would evolve in parallel.^[32] This method has been applied to natural experiments like the 1990s California minimum wage increase, where Card and Krueger compared fast-food employment changes in affected versus neighboring states, finding no disemployment effects contrary to neoclassical predictions.^[26] Validity hinges on the parallel trends assumption, verifiable via pre-treatment data trends, and extensions like triple differences address group-specific confounders.^[33] Recent refinements incorporate staggered adoption timing, as in Callaway and Sant'Anna's estimator, to handle heterogeneous treatment effects across units.^[34] Regression Discontinuity Design (RDD) identifies effects by comparing outcomes just above and below a deterministic cutoff in an assignment rule, treating the threshold as a local randomization point under continuity assumptions for potential outcomes.^[35] In natural experiments, this applies to eligibility rules, such as age-based scholarships in Sweden analyzed by Angrist and Lavy, where class size reductions at enrollment thresholds boosted student performance.^[26] Sharp RDD assumes perfect compliance at the cutoff, while fuzzy variants use the discontinuity in treatment probability as an instrument; bandwidth selection and density tests ensure no manipulation.^[36] Local randomization tests validate the design by checking balance in covariates near the cutoff.^[37] Instrumental Variables (IV) uses an exogenous instrument correlated with treatment but not directly with outcomes (beyond treatment effects) to address endogeneity, yielding local average treatment effects for compliers.^[7] Natural experiments provide instruments like draft lotteries, as in Angrist's Vietnam War studies, where lottery number predicted service but not earnings absent conscription.^[26] Requirements include relevance (first-stage strength, F-statistic >10) and exclusion (no direct paths), tested via overidentification in multiple instruments.^[38] Two-stage least squares implements estimation, with robustness to weak instruments via Anderson-Rubin tests.^[39] Interrupted Time Series (ITS) assesses impacts by modeling pre- and post-intervention trends in aggregate time-series data, attributing level or slope changes to the shock while adjusting for autocorrelation and seasonality.^[1] In public health natural experiments, such as evaluating tobacco control policies, ITS segmented regression estimates immediate effects and trend shifts, as in U.S. analyses post-1992 excise tax hikes showing reduced consumption.^[40] Robustness involves counterfactual simulations and controls for concurrent events; segmented Poisson models suit count data.^[41] These methods complement each other, with synthetic controls extending DiD/ITS for single-unit treatments by constructing weighted counterfactuals from donor pools.^[42] Empirical checks, like placebo tests on untreated periods, underpin credibility across techniques.^[43]

Notable Examples

Public Health and Policy Interventions

A foundational example in public health is John Snow's 1854 investigation of a cholera outbreak in London's Soho district, where he used spatial clustering of cases around the Broad Street pump to infer waterborne transmission. By petitioning for the pump handle's removal on September 8, 1854, Snow effectively intervened, after which new cases declined sharply, providing evidence of causality through this quasi-experimental variation in exposure. Snow further supported his hypothesis via a "grand experiment" comparing mortality rates between households supplied by the contaminated Southwark and Vauxhall Company versus the purer Lambeth Company, revealing tenfold higher cholera deaths in the former during the 1854 epidemic.^[44]^[3] In modern policy contexts, Scotland's Minimum Unit Pricing (MUP) policy for alcohol, implemented on May 1, 2018, at £0.50 per unit, serves as a natural experiment evaluated through difference-in-differences designs comparing Scotland to England and Wales. This intervention reduced alcohol purchases by an estimated 7.6% initially, with subsequent analyses showing 443 fewer alcohol-specific deaths and over 1,000 fewer hospital admissions than projected over the first five years, indicating causal impacts on consumption and health outcomes despite potential substitution effects.00497-X/fulltext)^[40] Smoking bans provide another class of policy interventions analyzed as natural experiments, with the short-term ban in Helena, Montana, from June 5 to December 2002, linked to a 40% drop in acute myocardial infarction admissions among local residents, attributed to diminished secondhand smoke exposure in public venues. While this small-scale case highlighted rapid health benefits, larger studies of comprehensive bans, such as Scotland's 2006 legislation, confirmed sustained reductions in cardiovascular events, with meta-analyses estimating 10-20% decreases in heart attack admissions post-implementation, underscoring the causal role of such policies in improving population health.^[45]^[46]

Labor and Economic Shocks

One prominent natural experiment in labor economics involves the Mariel Boatlift of 1980, when approximately 125,000 Cuban refugees unexpectedly arrived in Miami over five months, expanding the local labor force by about 7 percent, predominantly low-skilled workers. Economist David Card analyzed this exogenous immigration shock by comparing wage and employment trends for low-skilled workers in Miami against similar cities like Atlanta, Houston, and Los Angeles, using a difference-in-differences framework to isolate the influx's causal impact. His findings indicated no significant decline in wages or employment for native workers, challenging predictions from classical economic models that anticipated labor market displacement from a sudden supply increase.^[47] Subsequent research, however, has yielded mixed results; for instance, George Borjas reestimated the effects using alternative data and controls, reporting a 10-30 percent wage drop for high school dropouts, highlighting debates over data quality and model assumptions in interpreting such shocks. Another key example is the 1992 New Jersey minimum wage increase from $4.25 to $5.05 per hour, which David Card and Alan Krueger exploited as a natural experiment by contrasting fast-food restaurant employment in New Jersey with bordering Pennsylvania, where no change occurred. Through surveys of 410 establishments before and after the policy, they employed a difference-in-differences approach and found that employment rose by about 13 percent more in New Jersey, suggesting minimal or no disemployment effects and potential demand-side benefits from higher worker incomes.^[48] This contradicted traditional supply-demand predictions of job losses, but later audits and replications, including one using payroll data, indicated small employment reductions or null effects, underscoring sensitivities to measurement methods like self-reported versus administrative records.^[49] These cases illustrate how policy-induced or exogenous shocks enable causal inference on labor supply elasticity and market adjustments, though validity hinges on the exogeneity of the variation and robustness to alternative specifications.^[50] Economic shocks from commodity price fluctuations, such as oil booms, have also served as natural experiments for localized labor demand effects. In regions like the U.S. Great Plains, sudden drilling expansions increased male employment and wages by 5-10 percent without crowding out other sectors, as evidenced by comparisons to non-boom counties, revealing rigidities in worker mobility and short-term mismatches. Such analyses emphasize causal realism by leveraging geographic isolation to trace shock propagation, yet they require careful controls for confounding factors like migration responses.

Environmental and Biological Cases

The 1991 eruption of Mount Pinatubo in the Philippines injected approximately 20 million tons of sulfur dioxide into the stratosphere, forming aerosols that reflected sunlight and caused a global surface cooling of about 0.5°C for nearly two years.^[51] This event served as a natural experiment to assess volcanic forcing on climate sensitivity, with observations confirming model predictions of radiative cooling and testing feedback mechanisms like water vapor changes.^[52] Researchers compared pre- and post-eruption data, isolating the aerosol effect from anthropogenic trends, which revealed hemispheric asymmetries in temperature response and strengthened estimates of equilibrium climate sensitivity.^[53] In ecological contexts, the accidental introduction of the brown treesnake (Boiga irregularis) to Guam after World War II created a natural experiment demonstrating trophic cascades. The invasive predator extirpated nearly all native forest birds by the 1980s, leading to a documented explosion in arthropod populations, including spiders, which increased up to 3- to 10-fold in bird-absent forests compared to nearby snake-free islands.^[54] This variation allowed causal inference on top-down control, as bird removal indirectly boosted invertebrate herbivores and reduced herbivory on vegetation, highlighting the role of apex predators in maintaining ecosystem balance.^[54] A classic biological natural experiment is industrial melanism in the peppered moth (Biston betularia) during Britain's Industrial Revolution. Soot pollution darkened tree bark, increasing predation on light-colored moths by birds, which shifted melanic (dark) morph frequencies from under 5% in 1848 to over 95% by the 1890s in polluted Manchester.^[55] Post-1956 clean air regulations reversed this, with melanic forms declining to rarity by the 1980s, as confirmed by recapture studies showing survival advantages of cryptic coloration matching cleaner lichens.^[56] Bernard Kettlewell's 1950s field releases quantified predation differentials, providing empirical support for visual selection driving the shift without genetic evidence of mutation rates exceeding background levels.^[55] In evolutionary biology, variation in predation pressure across Trinidadian streams has enabled natural experiments on guppy (Poecilia reticulata) life-history evolution. High-predation sites with pike cichlids select for earlier maturity, smaller size at reproduction, and higher offspring numbers, contrasting low-predation streams where guppies allocate more to growth and fewer, larger young.^[57] David Reznick's transplants of guppies to predator-free upstream sites in the 1980s-2000s induced rapid shifts—within 4-11 years (8-30 generations)—toward low-predation traits, with heritable changes in age/size at maturity evolving 2.2-3.5 times faster than neutral expectations.^[58] These parallel responses across streams isolated predation as the exogenous driver, influencing ecosystem processes like algal biomass via evolved foraging behaviors.^[58]

Advantages

Empirical Rigor in Uncontrolled Settings

Natural experiments attain empirical rigor in uncontrolled settings by harnessing exogenous variations—such as abrupt policy shifts, natural disasters, or geographic discontinuities—that plausibly mimic random assignment to treatment and control groups, thereby facilitating causal identification without researcher manipulation.^[12]^[22] This leverages real-world events where treatment exposure arises independently of individual choices or confounders, reducing endogeneity and enabling comparisons between otherwise comparable units, as seen in analyses of sudden immigration restrictions or lottery-based allocations.^[59] Such designs prioritize internal validity through testable assumptions, like the absence of anticipation effects or spillovers, which, when validated via placebo tests or robustness checks, yield estimates robust to unobserved heterogeneity.^[13] Quasi-experimental techniques further bolster this rigor by statistically adjusting for temporal and fixed differences; for instance, difference-in-differences exploits pre-treatment parallel trends between groups, assuming that any common shocks affect both equally post-treatment, while regression discontinuity uses sharp cutoffs (e.g., age eligibility thresholds) to ensure local randomization around the margin.^[60]^[61] These methods, rooted in econometric and epidemiological traditions, outperform naive observational approaches by explicitly addressing threats like omitted variable bias, with falsification strategies—such as examining non-equivalent outcomes—serving to corroborate the exogeneity of the variation.^[62] In practice, this has produced policy-relevant findings, such as the labor market effects of minimum wage hikes in specific locales, where the uncontrolled rollout provided credible counterfactuals absent in lab settings.^[63] The uncontrolled context, while introducing potential external confounders, paradoxically enhances rigor by embedding analysis in authentic behavioral dynamics, avoiding the artificiality of lab manipulations that may distort incentives or scale.^[22] Researchers mitigate remaining validity threats through sensitivity analyses, including synthetic controls or instrumental variables drawn from the natural shock itself, ensuring inferences remain grounded in empirical patterns rather than unverified modeling assumptions.^[59] This framework has proven particularly valuable in fields like public economics, where ethical constraints preclude randomization of interventions like tax reforms, yet yields precise effect sizes comparable to controlled trials under verified conditions.^[12]

Complement to Randomized Trials

Natural experiments complement randomized controlled trials (RCTs) by providing causal evidence in contexts where ethical constraints, logistical challenges, or the scale of interventions preclude randomization, such as large-scale policy reforms or historical events.^[1] Unlike RCTs, which rely on researcher-controlled random assignment to minimize confounding, natural experiments leverage exogenous shocks—like sudden regulatory changes or natural disasters—that mimic randomization by affecting treatment and control groups comparably, absent manipulation by investigators.^[8] This approach, rigorously formalized in the 1990s by economists Joshua Angrist and Guido Imbens, enables estimation of local average treatment effects under assumptions of instrument validity and exclusion restrictions, yielding inferences akin to those from RCTs when those assumptions hold.^[64]^[65] In fields like economics and public health, natural experiments address RCTs' limitations in external validity and scope; for instance, RCTs often operate in constrained settings with short horizons, potentially missing general equilibrium effects or long-term dynamics observable in real-world policy shocks, such as the 1994 North American Free Trade Agreement's impact on labor markets.^[66]^[13] They facilitate analysis of interventions at population levels, like universal health policy rollouts, where randomizing entire societies would be infeasible or unethical, thus broadening the evidentiary base for causal claims beyond RCT-dominated domains.^[67] Empirical work using natural experiments has, for example, quantified returns to schooling via Vietnam War draft lotteries, offering credible estimates that inform broader policy without the artificiality of lab trials.^[68] Despite these strengths, natural experiments do not supplant RCTs but augment them, as both methods demand scrutiny of threats like selection bias or spillover effects, though natural experiments often require stronger reliance on contextual knowledge to validate exogeneity.^[69] Their integration with RCT evidence enhances robustness; for instance, findings from natural experiments on minimum wage hikes have been cross-validated against smaller-scale RCT analogs, revealing consistent labor supply responses while highlighting scale-dependent elasticities missed in controlled studies.^[70] This complementarity underscores a pluralistic empirical strategy, prioritizing designs that best match the question's real-world constraints over dogmatic adherence to any single method.^[71]

Criticisms and Limitations

Threats to Causal Validity

Natural experiments depend on the plausibility of identifying assumptions, such as the exogeneity of the treatment variation, to support causal inferences; violations of these assumptions can introduce bias and invalidate conclusions.^[72] For instance, if the exogenous shock is correlated with unobserved confounders—such as underlying economic conditions influencing both policy changes and outcomes—the estimated treatment effect may reflect these confounders rather than the intervention itself.^[73] This threat is particularly acute in observational settings where researchers lack direct control over treatment assignment, unlike randomized controlled trials.^[1] A common violation occurs when the parallel trends assumption fails in difference-in-differences designs, a frequent method in natural experiments; this assumption requires that treatment and control groups would have followed similar trajectories absent the intervention, but differential pre-treatment trends due to selection or anticipation effects can bias results.^[32] Empirical tests, such as placebo analyses on pre-treatment periods, can detect such violations, yet they do not guarantee absence of unobserved time-varying confounders.^[37] Spillover effects represent another internal validity threat, where the treatment influences control units through migration, trade, or behavioral responses, violating the stable unit treatment value assumption (SUTVA) and leading to underestimation of impacts.^[74] Heterogeneous treatment effects further complicate validity, as the instrument or shock may induce varying responses across subgroups, with local average treatment effects (LATE) in instrumental variable approaches applying only to compliers rather than the full population.^[31] Reverse causality or anticipation—where agents foresee and respond to the impending shock—can also confound estimates, as seen in policy evaluations where behavioral adjustments precede the event.^[75] Measurement error in treatment exposure or outcomes exacerbates these issues, particularly in administrative data common to natural experiments.^[1] External validity threats, while distinct, intersect with causal claims when results fail to generalize beyond the specific context of the shock; for example, a natural experiment exploiting a regional disaster may not capture nationwide policy effects due to unique local factors.^[76] Sensitivity analyses, such as synthetic controls or robustness checks to alternative assumptions, are essential to assess these threats, though they cannot fully eliminate reliance on untestable conditions.^[77] Overall, while natural experiments offer quasi-random variation, their causal validity hinges on the credibility of the exclusion restriction and absence of confounding pathways, demanding rigorous falsification and theoretical justification.^[69]

Assumptions and Interpretation Challenges

Natural experiments require stringent assumptions to enable causal inference, as they approximate randomization through exogenous variation but lack direct control over assignment. A core assumption is the exogeneity of the shock or intervention, whereby the natural event must affect outcomes solely through the treatment mechanism and not directly via other channels, akin to an instrumental variable's exclusion restriction.^[37] This demands that the variation be "as-if random," uncorrelated with unobserved confounders that influence the outcome.^[9] In difference-in-differences designs, a prevalent method within natural experiments, the parallel trends assumption is essential: without treatment, outcome trends for treated and control groups would evolve similarly over time.^[32] Interpreting these designs faces challenges from untestable assumptions, as exogeneity and parallel trends cannot always be directly verified and may fail if agents anticipate the shock or if spillover effects occur across groups.^[1] Unmeasured confounding poses a persistent threat, where omitted variables correlated with both the shock and outcome undermine causal claims, despite efforts like pre-trends testing or placebo checks.^[43] Selective exposure further complicates analysis, as individuals or units may self-select into treatment based on unobservables, violating randomization-like conditions.^[1] Generalizability remains limited, with effects often context-specific and non-replicable due to the uniqueness of natural shocks, raising doubts about external validity beyond the studied setting.^[76] Researchers must also contend with heterogeneous treatment effects, where average impacts mask subgroup variations, and potential reverse causality if outcomes influence shock exposure, though the latter is rarer in truly exogenous cases.^[75] These issues necessitate robustness checks, such as synthetic controls or multiple event studies, to bolster credibility, yet reliance on such quasi-experimental tools underscores their inferiority to randomized trials when feasible.^[32]

Recent Developments

Policy-Focused Applications

Natural experiments provide a framework for evaluating policy interventions by exploiting exogenous variations arising from legislative changes, regulatory shifts, or abrupt implementations that mimic randomization. In policy analysis, these designs enable causal inference on outcomes such as employment, health, or environmental quality without the ethical issues of withholding treatments in randomized trials. For example, difference-in-differences approaches compare treated and control groups before and after a policy shock, assuming parallel trends absent the intervention.00024-5/fulltext)^[78] Recent applications in public health have assessed multi-sector policies for chronic disease management. The NEXT-D Study Group analyzed natural experiments in diabetes prevention, using pragmatic data from policy variations across U.S. regions to compare outcomes like screening rates and glycemic control against counterfactual scenarios. Similarly, evaluations of place-based interventions, such as urban green space expansions or transport subsidies, have quantified impacts on physical activity and obesity by leveraging policy rollouts in specific locales versus non-affected areas. These studies, often post-2020, highlight natural experiments' role in informing scalable interventions amid resource constraints.^[79]^[78] In environmental and economic policy, quasi-natural experiments have examined trade-offs from regulatory reforms. The 2020 Yangtze River Basin protection policy in China, prohibiting heavy industry in protected zones, was evaluated via difference-in-differences on panel data from 2016–2021, revealing reductions in pollution but heterogeneous economic effects across urban levels. In criminal justice, New York State's 2009 drug law reforms, eliminating mandatory minimums for certain felonies, served as a natural experiment to estimate changes in sentencing lengths and recidivism rates, with findings indicating modest reductions in prison populations without crime spikes. Such applications underscore natural experiments' utility for real-time policy feedback, though results depend on robust identification of exogenous shocks.^[80]^[81]

Methodological Innovations

One significant methodological innovation in natural experiments is the synthetic control method (SCM), introduced by Abadie, Diamond, and Hainmueller in 2010, which constructs a counterfactual outcome trajectory for a treated unit by optimally weighting a combination of untreated control units to match pre-intervention predictors and outcomes.^[82] This approach addresses limitations of simple difference-in-differences in comparative case studies, such as California's Proposition 99 tobacco control program in 1988, by minimizing researcher discretion in control selection and improving fit to observed pre-treatment trends.^[83] SCM has been extended to generalized forms, enabling analysis of multiple treated units, as demonstrated in applications estimating health effects of extreme weather events like hurricanes, where traditional controls fail due to spatial spillovers.^[84] Parallel advances have refined difference-in-differences (DiD) estimators for natural experiments involving staggered treatment adoption, where policies roll out across units at different times, such as state-level minimum wage increases. Callaway and Sant'Anna's 2021 framework provides identification and estimation for average treatment effects on the treated under heterogeneous timing, using group-time average treatment effects and aggregation to avoid biases from negative weighting or heterogeneous effects that plague two-way fixed effects models.^[85] This method incorporates never-treated units as anchors for parallel trends assumptions and supports robust inference via bootstrap procedures, enhancing applicability to policy shocks like the U.S. Affordable Care Act's Medicaid expansions starting in 2014.^[86] Recent frameworks emphasize mixed-methods integration for natural experiments in population health, as outlined in the 2025 UK Medical Research Council and National Institute for Health and Care Research guidance, which advocates combining quantitative quasi-experimental designs (e.g., interrupted time series, regression discontinuity) with qualitative process tracing and system mapping to elucidate mechanisms and contextual factors influencing causal pathways.^[87] These innovations incorporate risk-of-bias tools like ROBINS-I for non-randomized studies and realist synthesis for complex interventions, addressing interpretation challenges in uncontrolled settings by prioritizing empirical validation of assumptions over reliance on untestable parallels.^[88] Such hybrid approaches have facilitated evaluations of interventions like urban green space expansions, where qualitative data reveals assignment mechanisms absent in purely quantitative analyses.^[89]

References

[1]
Natural Experiments: An Overview of Methods, Approaches, and ...
Natural experiment (NE) approaches are attracting growing interest as a way of providing evidence in such circumstances.
[2]
Natural Experiments - NASA ADS
I define a natural experiment as a research study where the treatment assignment mechanism (i) is neither designed nor implemented by the researcher, (ii) is ...
[3]
John Snow, Cholera, the Broad Street Pump; Waterborne Diseases ...
John Snow conducted pioneering investigations on cholera epidemics in England and particularly in London in 1854 in which he demonstrated that contaminated ...
[4]
Remembering Dr. John Snow on the sesquicentennial of his death
In Snow's “grand experiment,” he compared cholera death rates in households served by 2 rival water companies. Basically, 1 of these companies obtained its ...
[5]
[PDF] Instrumental Variables and the Search for Identification
In essence, the combination of school start age policies and compulsory schooling laws creates a natural experiment in which children are compelled to attend ...<|separator|>
[6]
Natural experiments: A Nobel Prize awarded research design ... - NIH
Apr 28, 2022 · Natural experiments allow the retrospective and prospective evaluation of policies, interventions or programs in real-world settings.
[7]
[PDF] Natural Experiments in Macroeconomics
We discuss and compare the use of natural experiments across these different applications and summarize what they have taught us about such diverse subjects as ...
[8]
[PDF] answering causal questions using observational data - Nobel Prize
Oct 11, 2021 · The underlying variation (i.e., the natural experiment) can come from policy changes, administrative rules, naturally occurring random variation ...
[9]
https://oxfordre.com/business/display/10.1093/acrefore/9780190224851.001.0001/acrefore-9780190224851-e-302
[10]
Introduction (Chapter 1) - Natural Experiments in the Social Sciences
Anthropologists, geographers, and historians have used natural experiments to study topics ranging from the effects of the African slave trade to the long-run ...
[11]
[PDF] AngristPischke.pdf - Econometrics at the university of illinois
Design-based studies typically feature either real or natural experiments and are distinguished by their prima facie credibility and by the attention ...
[12]
[PDF] 8 Using Natural Experiments to Provide “Arguably Exogenous ...
When we say that experimental assignment is exogenous, we mean that it is beyond any possible manipu- lation by the participants themselves, so that membership ...
[13]
Conceptualising natural and quasi experiments in public health - PMC
Feb 11, 2021 · In this paper we argue for a clearer conceptualisation of natural experiment studies in public health research, and present a framework to ...
[14]
[PDF] Experiment Design
Sep 17, 2015 · experimental and control conditions are determined by nature or by other factors outside the control of the investigators. SOURCES ...
[15]
[PDF] NATURAL EXPERIMENTS - Center for Public Health Law Research
Most changes in laws and regulations affecting population health represent natural experiments, in which scientists do not control when and where these changes ...
[16]
[2002.00202] Natural Experiments - arXiv
Feb 1, 2020 · ... natural experiments and at the same time distinguish them from randomized controlled experiments. I define a natural experiment as a ...Missing: distinction | Show results with:distinction
[17]
[PDF] Learning in Games and the Interpretation of Natural Experiments12
Natural experiments are widely used in economics to estimate treatment effects in non-experimental settings. As is well known, randomness and explanatory power ...
[18]
7. John Snow and the Natural Experiment
John Snow in the mid-1800s when he investigated the relationship between drinking contaminated water and the incidence of cholera. A natural experiment is ...
[19]
Principles of Epidemiology | Lesson 1 - Section 2 - CDC Archive
To confirm that the Broad Street pump was the source of the epidemic, Snow gathered information on where persons with cholera had obtained their water.
[20]
John Snow Hunts the Blue Death | Science History Institute
Mar 8, 2022 · John Snow Hunts the Blue Death ... In showing that cholera spreads through tainted water, an English doctor helped lay epidemiology's foundations.
[21]
John Snow, Henry Whitehead, the Broad Street pump, and the ...
His hypothesis that cholera was spread by contaminated water was tested by the 'Broad Street' epidemic of 1854. Snow quickly traced the water used in the houses ...Review · Cholera · Conclusion
[22]
Capitalizing on Natural Experiments to Improve Our Understanding ...
Natural experiments have a long history in public health research, the most famous being John Snow's 1853 study of cholera in Londoners supplied with drinking ...
[23]
John Snow: The First Hired Gun? | American Journal of Epidemiology
Snow also presented data from the 1854 cholera outbreak as the basis for his belief that epidemic diseases were transmitted by water, not air. Although the data ...
[24]
The Credibility Revolution in Empirical Economics
They implicitly use state-level variation in education spending as a natural experiment: aggrega- tion of individual data up to the cohort/state level is an ...
[25]
The Credibility Revolution in Empirical Economics: How Better ...
Design-based studies typically feature either real or natural experiments and are distinguished by their prima facie credibility and by the attention ...
[26]
[PDF] Natural experiments help answer important questions - Nobel Prize
A unique event in the history of the US gave rise to a natural experiment, which David Card used to investigate how immigration affects the labour market ...
[27]
The Prize in Economic Sciences 2021 - Popular science background
This year's Laureates – David Card, Joshua Angrist and Guido Imbens – have shown that natural experiments can be used to answer central questions for society.
[28]
Natural Experiments in the Social Sciences
Part I - Discovering natural experiments. pp 39-40 · 2 - Standard natural experiments. pp 41-62 · 3 - Regression-discontinuity designs. pp 63-86 · 4 - Instrumental ...
[29]
6. Natural Experiments and Quasi-Experiments - APSA Educate
We have a natural experiment when variation in the independent variable is randomly assigned, but not by the researcher. One example of a natural experiment was ...<|separator|>
[30]
Historical Natural Experiments: Bridging Economics and Economic ...
Feb 13, 2020 · We argue that the historical natural experiment represents a methodological bridge between economic history and other fields.
[31]
[PDF] natural and quasi- experiments in economics
describes the sources of exogenous variation in natural experiments and other studies. Section 8 describes instrumental variables methods that have been ...
[32]
9 Difference-in-Differences - Causal Inference The Mixtape
This is the story of how John Snow convinced the world that cholera was transmitted by water, not air, using an ingenious natural experiment (Snow 1855).
[33]
[PDF] Week 4: Difference-in-differences
experiments: difference-in-difference (DiD), regression discontinuity (RD), and instrumental variables (IV). 3. Page 4. Big picture II. Just to avoid confusion ...
[34]
Natural Experiments: An Overview of Methods, Approaches, and ...
Mar 20, 2017 · Natural Experiments: An Overview of Methods, Approaches, and Contributions to Public Health Intervention Research.<|control11|><|separator|>
[35]
[PDF] Regression Discontinuity Designs in Economics - Princeton University
Regression Discontinuity (RD) designs were first introduced by Donald L. Thistlethwaite and Donald T. Campbell. (1960) as a way of estimating treatment.
[36]
Regression Discontinuity for Causal Effect Estimation in Epidemiology
The regression discontinuity design can be thought of as an extension of instrumental variable analysis, in circumstances where an exogenous source of variation ...
[37]
Chapter 4 Natural Experiments | Statistical Tools for Causal Inference
Both Difference In Differences and Regression Discontinuity Designs have been actually developed outside of economics, mostly in education research.
[38]
Chapter 11 Difference in Differences | Econometrics for ... - Bookdown
In this case, there are three additional empirical strategies typically use: Difference in Differences; Instrumental Variables; Regression Discontinuity. Today, ...
[39]
Natural Experiments in Macroeconomics - ScienceDirect.com
Natural experiments have been used to verify underlying assumptions of conventional models, quantify specific model parameters, and identify mechanisms.
[40]
Using natural experiments to evaluate population health ... - The BMJ
Mar 28, 2025 · Natural experiments are widely used to evaluate the impacts on health of changes in policies, infrastructure, and services.
[41]
Conceptualising natural and quasi experiments in public health
Feb 11, 2021 · In this paper we argue for a clearer conceptualisation of natural experiment studies in public health research, and present a framework to improve their design ...
[42]
https://www.actuary.org/wp-content/uploads/2024/12/risk-paper-natural-experiments.pdf
[43]
Natural experiment methodology for research: a review of how ...
A natural experiment occurs when a particular intervention has been implemented but the circumstances surrounding the implementation are not under the control ...<|control11|><|separator|>
[44]
Cholera, John Snow and the Grand Experiment
Aug 18, 2010 · Snow is credited with the discovery that cholera is transmitted through sewage-tainted water. His map of London's Soho region is often ...
[45]
Archive: Six-month public smoking ban slashes heart attack rate in ...
Apr 1, 2003 · The study took place in Helena, Montana, where a smoke-free ordinance went into effect June 5, 2002 and was suspended in a legal challenge ...
[46]
A review of the evidence on smoking bans and incidence of heart ...
We update an earlier review of smoking bans and heart disease, restricting attention to admissions for acute myocardial infarction.Missing: experiment | Show results with:experiment
[47]
[PDF] The Impact of the Mariel Boatlift on the Miami Labor Market David ...
Jun 27, 2007 · The experiences of the Miami labor market in the aftermath of the Marie1. Boatlift provide a natural experiment with which to evaluate the ...
[48]
[PDF] Minimum Wages and Employment: A Case Study of the Fast-Food ...
On April 1, 1992, New Jersey's minimum wage rose from $4.25 to $5.05 per hour. To evaluate the impact of the law we surveyed 410 fast-food restaurants in.
[49]
Minimum Wages and Employment: A Case Study of the Fast Food ...
On April 1, 1992 New Jersey's minimum wage increased from $4.25 to $5.05 per hour. To evaluate the impact of the law we surveyed 410 fast food restaurants ...
[50]
https://www.aeaweb.org/articles?id=10.1257/aer.84.4.772
[51]
The atmospheric impact of the 1991 Mount Pinatubo eruption
Nov 6, 1999 · Effects on climate were an observed surface cooling in the Northern Hemisphere of up to 0.5 to 0.6°C, equivalent to a hemispheric-wide reduction ...
[52]
Radiative Climate Forcing by the Mount Pinatubo Eruption - Science
The volcanic aerosols caused a strong cooling effect immediately; the amount of cooling increased through September 1991 as shortwave forcing increased.
[53]
[PDF] Mount Pinatubo as a Test of Climate Feedback Mechanisms
The Mount Pinatubo eruption was a short-lived shock to the atmosphere, used to study climate, test models, and examine impacts of climate change, and to ...
[54]
'Natural experiment' Demonstrates Top-Down Control of Spiders by ...
Sep 7, 2012 · We use a unique natural experiment, the extirpation of insectivorous birds from nearly all forests on the island of Guam by the invasive brown tree snake.Missing: notable | Show results with:notable
[55]
The peppered moth and industrial melanism: evolution of a ... - Nature
Dec 5, 2012 · Overall, these experiments suggested that the melanic peppered moth had an advantage of up to 2 to 1 over the typical form in industrial ...
[56]
Selective bird predation on the peppered moth: the last experiment ...
Melanism in the peppered moth Biston betularia led to the earliest measurements of natural selection on a Mendelian locus in the wild [1,2]. Rapid nineteenth ...
[57]
Experimental studies of evolution in guppies: a model for ... - PubMed
Experimental studies of evolution in guppies: a model for understanding the evolutionary consequences of predator removal in natural communities. Mol Ecol ...
[58]
Local adaptation in Trinidadian guppies alters ecosystem processes
Feb 23, 2010 · Our findings demonstrate that evolution can significantly affect both ecosystem structure and function.
[59]
[PDF] Natural and Guasi-Experiments - UChicago Voices
Using research designs patterned after randomized experiments, many recent economic studies examine outcome measures for treatment groups and comparison ...
[60]
[PDF] Using natural experimental studies to guide public health action
We discuss the growing importance of evaluating natural experiments and their distinctive contribution to the evidence for public health policy. We contrast the ...
[61]
[PDF] EXPERIMENTAL AND QUASI-EXPERIMENTAL DESIGNS FOR ...
tively, such situations can be regarded as quasi-experimental designs. (Campbell &. StanleS 1,963, p. 34) ... Natural experiments have recently gained a high ...
[62]
Leveraging natural experiments to evaluate interventions in learning ...
Leveraging natural experiments to evaluate interventions in learning health systems ... internal validity. For example, they may rely on strong and unrealistic ...
[63]
Internal and External Validity in Economics Research - jstor
We define internal validity as the ability of a researcher ... natural experiments are limited to what natu ral or external forces provide the researcher.
[64]
Nobel-winning 'natural experiments' approach made economics ...
Oct 13, 2021 · Nobel-winning 'natural experiments' approach made economics more robust. Joshua Angrist, Guido Imbens and David Card share the prize for finding ...
[65]
Imbens' work ignited an empirical revolution in economics
Oct 11, 2021 · Natural experiments. In an innovative study from 1994, Angrist and Imbens created a framework to show what conclusions about causation can be ...
[66]
[PDF] Complements and Alternatives to RCTs in Evaluation in Development
In this sense, structural models and RCTs complement each other. The ... Natural “natural experiments" in economics. Journal of Economic Literature ...
[67]
Using natural experimental studies to guide public health action
Patient consent for publication Not required. Provenance and peer review Not commissioned; externally peer reviewed. Data availability statement There are no ...
[68]
[PDF] Natural experiments help answer important questions - Nobel Prize
Joshua Angrist and Guido Imbens thus showed exactly what conclusions about cause and e ect can be drawn from natural experiments. Their analysis is also ...Missing: RCTs | Show results with:RCTs
[69]
26.6 Natural Experiments | A Guide on Data Analysis - Bookdown
... natural experiments provide an indispensable tool for causal inference, particularly when RCTs are impractical, unethical, or prohibitively expensive. Key ...<|separator|>
[70]
The 2021 Nobel Prize in Economic Sciences - Oxera
Nov 26, 2021 · A fundamental difference between natural experiments and RCTs is the researcher's ability to control who receives the treatment. In an RCT ...
[71]
Beyond Causality: Additional Benefits of Randomized Controlled ...
RCTs Can Study the Questions You Want to Study, Rather Than the Ones Nature Permits. Natural experiments can be used to obtain convincing evidence of causal ...
[72]
Improving Causal Inference: Strengths and Limitations of Natural ...
threats to valid causal inference in observational settings can arise. The author proposes a continuum of plausibility for natural experiments, defined by ...
[73]
Chapter 6 Threats to the validity of Causal Inference
It is classical to make a disctinction between two threat and one specific set of problems: Threats to internal validity: these are the threats that vitiate the ...
[74]
Causal assumptions and causal inference in ecological experiments
Sep 15, 2021 · We review four assumptions required to infer causality from experiments and provide design-based and statistically based solutions for when these assumptions ...
[75]
Commentary: Natural and Unnatural Experiments in Epidemiology
Another limitation of natural experiments more generally, compared with standard RCTs, is that researchers do not have control over the nature of the exposure ...
[76]
Natural Experiments: Missed Opportunities for Causal Inference in ...
Feb 15, 2024 · Natural experiments are challenging to find, provide information about only specific causal effects, and involve assumptions that are difficult ...
[77]
[PDF] What are 'natural experiments' - The Scottish Government
Natural experiments are observational studies which can be undertaken to assess the outcomes and impacts of policy interventions.<|separator|>
[78]
Natural experiments for the evaluation of place-based public health ...
Jun 22, 2023 · One of the most famous examples of a place-based natural experiment in public health is that of John Snow and the Broad Street pump, where he ...
[79]
Evaluating Diabetes Health Policies Using Natural Experiments
This manuscript describes the NEXT-D Study group's multi-sector natural experiments in areas of diabetes prevention or control as case examples to illustrate ...
[80]
A holistic approach to evaluating environmental policy impact using ...
... policy as a quasi-natural experiment. Drawing on urban-level panel data from the Yangtze River Basin between 2016 and 2021, we demonstrate that the DID ...
[81]
Natural Experiment in Reform: Analyzing Drug Policy Change In ...
This study evaluated reforms to New York State's drug laws that removed mandatory minimum sentences for defendants facing a range of felony drug and property ...
[82]
Synthetic Control Methods for Comparative Case Studies
Building on an idea in Abadie and Gardeazabal (2003), this article investigates the application of synthetic control methods to comparative case studies.
[83]
[PDF] Synthetic Control Methods For Comparative Case Studies
Building on an idea in Abadie and Gardeazabal (2003), this article investigates the application of synthetic control methods to comparative case studies. We ...
[84]
Using the Generalized Synthetic Control Method to Estimate the ...
Nov 1, 2022 · We illustrate the use of generalized synthetic control to leverage natural experiments to quantify the health impacts of extreme weather events.
[85]
Difference-in-Differences with multiple time periods - ScienceDirect
In this article, we provide a unified framework for average treatment effects in DiD setups with multiple time periods, variation in treatment timing,
[86]
[PDF] Difference-in-Differences with Multiple Time Periods
Dec 1, 2020 · In this article, we provide a unified framework for average treatment effects in DiD setups with multiple time periods, variation in treatment ...
[87]
Using natural experiments to evaluate population health and health ...
Mar 28, 2025 · Natural experiments are widely used to evaluate the impacts on health of changes in policies, infrastructure, and services.
[88]
https://doi.org/10.1136/bmj.i4919
[89]
https://doi.org/10.1186/s12874-022-01790-8