Relative strength index
The Relative Strength Index (RSI) is a momentum oscillator used in technical analysis to measure the speed and magnitude of recent price changes in financial assets, helping traders identify overbought or oversold conditions.[1] Developed by American engineer and trader J. Welles Wilder Jr. in 1978 and introduced in his seminal book New Concepts in Technical Trading Systems, the RSI quantifies the relative strength of upward versus downward price movements over a specified period, typically 14 days.[2][3] The indicator is calculated using the formula RSI = 100 - (100 / (1 + RS)), where RS (Relative Strength) is the average gain of up periods divided by the average loss of down periods, smoothed over time to reflect ongoing momentum.[1] It produces values ranging from 0 to 100, with readings above 70 signaling potential overbought conditions that may precede a price reversal or pullback, and readings below 30 indicating oversold conditions that could signal an upcoming rebound.[4] While the standard thresholds are 70 and 30, these can be adjusted for different market volatilities or asset classes, such as using 80 and 20 in strongly trending markets to reduce false signals.[5] In practice, the RSI is widely applied across stocks, forex, commodities, and cryptocurrencies to gauge trend strength, spot divergences (where price and RSI move in opposite directions, hinting at reversals), and confirm other technical signals, though it performs best in ranging or sideways markets rather than strong trends where it may remain overextended.[6] Despite its popularity since the late 1970s, limitations include its sensitivity to the chosen lookback period and potential for whipsaws in volatile environments, prompting traders to combine it with volume indicators or moving averages for more robust analysis.[1]Introduction
Definition and Purpose
The Relative Strength Index (RSI) is a momentum oscillator used in technical analysis to measure the speed and magnitude of recent price changes in financial instruments, such as stocks, commodities, or currencies.[2][7] It provides a bounded scale from 0 to 100, where values near 100 indicate strong upward momentum and values near 0 suggest weak or downward momentum.[2][8] The primary purpose of the RSI is to identify potential overbought or oversold conditions in the market, signaling possible price reversals or trading opportunities.[2][7] Traditionally, an RSI reading above 70 indicates an overbought asset, which may be due for a pullback, while a reading below 30 suggests an oversold condition, potentially preceding a rebound.[8][7] These thresholds help traders gauge the relative strength of price movements compared to historical norms over a given period.[2] At its core, the RSI compares the average magnitude of gains to the average magnitude of losses over a specified look-back period, normalizing this ratio to produce the oscillator value.[7] For instance, in stock trading, a high RSI might reveal that a share price has risen sharply relative to its recent performance, indicating the asset could be overextended and at risk of correction.[2] This assessment aids investors in timing entries or exits without relying solely on price direction.[8]Historical Development
The Relative Strength Index (RSI) was developed by J. Welles Wilder Jr., a mechanical engineer who transitioned into technical analysis,[9] and first published in June 1978 in an article in Commodities magazine (now known as Futures magazine).[10] Wilder detailed the indicator in his seminal book New Concepts in Technical Trading Systems, released later that year by Trend Research, where RSI appeared alongside other innovations he created, such as the Parabolic SAR, Average True Range (ATR), and Average Directional Index (ADX). These tools formed a cohesive suite designed to help traders navigate complex market dynamics through objective, quantifiable signals.[11] Wilder's work emerged amid the heightened volatility of the 1970s commodity futures markets, characterized by frequent limit moves, price gaps, and sharp fluctuations driven by geopolitical events, oil crises, and supply disruptions.[12] At the time, traditional technical methods often failed to capture the full extent of intraday and overnight price swings in commodities like grains, metals, and energy products, prompting Wilder to devise RSI as a momentum-based oscillator to measure the speed and magnitude of price changes more reliably.[1] His background in engineering influenced the indicator's emphasis on smoothed averages and relative comparisons, aiming to provide traders with a standardized way to assess internal market strength without relying solely on price direction. Following its introduction, RSI saw rapid early adoption among commodity and futures traders seeking to identify potential reversals in volatile environments, and it soon extended to stock market analysis as technical trading gained broader acceptance in the late 1970s and 1980s.[1] Wilder himself made no significant updates to the core RSI methodology in subsequent works, preserving its original 14-period default and calculation approach as the standard. By the 1990s, the indicator had become a staple in professional trading software platforms, facilitating its integration into algorithmic and retail trading systems worldwide. Wilder passed away on April 18, 2021.[9]Calculation
Core Formula
The Relative Strength Index (RSI) is computed using a standard period of 14 days or bars, as originally specified by its developer J. Welles Wilder Jr.[1] To calculate RSI, first determine the gains and losses for each period based on closing prices: a gain is the positive difference between consecutive closes (or zero if the change is negative or zero), and a loss is the absolute value of the negative difference (or zero if the change is positive or zero).[13] For the initial RSI value, compute the average gain and average loss as the simple arithmetic mean of the first 14 gains and losses, respectively: \text{Average Gain} = \frac{\sum_{i=1}^{14} \text{Gain}_i}{14}, \quad \text{Average Loss} = \frac{\sum_{i=1}^{14} \text{Loss}_i}{14}. Subsequent values use Wilder's smoothing method, an exponential moving average variant with a smoothing factor of $1/14: \text{Average Gain}_t = \frac{(\text{Previous Average Gain} \times 13) + \text{Current Gain}}{14}, \text{Average Loss}_t = \frac{(\text{Previous Average Loss} \times 13) + \text{Current Loss}}{14}. This approach weights recent data while incorporating historical averages.[1][13] The relative strength (RS) is then the ratio of the average gain to the average loss: \text{RS} = \frac{\text{Average Gain}}{\text{Average Loss}}. The RSI is derived from RS using the formula: \text{RSI} = 100 - \frac{100}{1 + \text{RS}}. Special cases handle division by zero: if the average loss is zero, RSI is set to 100; if the average gain is zero, RSI is set to 0.[1][13] As an illustrative example, consider the following gains and losses over 14 periods (derived from hypothetical closing price changes):| Period | Gain | Loss |
|---|---|---|
| 1 | 2 | 0 |
| 2 | 0 | 1 |
| 3 | 3 | 0 |
| 4 | 0 | 2 |
| 5 | 1 | 0 |
| 6 | 0 | 1 |
| 7 | 4 | 0 |
| 8 | 0 | 3 |
| 9 | 2 | 0 |
| 10 | 0 | 1 |
| 11 | 3 | 0 |
| 12 | 0 | 1 |
| 13 | 1 | 0 |
| 14 | 0 | 2 |