The relative strength index (RSI) is a mathematically derived indicator used in the technical analysis of financial instruments. Classified as an oscillator, RSI attempts to quantify pricing momentum through the examination of a given security's realized gains and losses. Traders and investors use the RSI calculation to determine whether or not a security can be considered overbought or oversold.
RSI is a normalized technical indicator. Indicators that are normalized have pre-defined upper and lower values, or exist on a scale. In the case of RSI, the scale ranges from a low value of zero to a high value of 100. Using this scale, traders can determine whether or not a security is currently being overbought or oversold.
Generally, an RSI value of 30 or less is a signal that a security is undervalued, and may be poised for a rally in price. RSI calculations of 70 or greater are viewed as a sign that a security is becoming overvalued and a subsequent drop in price may be forthcoming.
In 1978, technical analyst and trader Welles Wilder Jr. first introduced the concept of RSI in his book "New Concepts In Technical Trading Systems. Since then, RSI has been applied to a wide variety of financial instruments including those actively traded on the futures, equities and forex markets . In addition, RSI has been adapted to trading practices encompassing many different time frames, ranging from short-term intraday trading to long-term investing.
Among traders and investors, RSI remains one of the most popular technical indicators in use today. Typically, it's used in concert with trend lines, assorted momentum oscillators, predetermined support and resistance levels or chart patterns in order to place the price action of a security into context. Accordingly, RSI is often a contributor in the identification of indicator confirmation or divergence.
However, RSI does face a few challenges. Trending markets can provide RSI calculations that remain in overbought and oversold areas for long periods of time. This is problematic in that false indications of market reversal are created, prompting traders to adopt a contrarian position within the trending market. When used on a short-term intraday basis, a momentary spike in pricing volatility can skew the RSI calculation itself, and produce an inaccurate value. As a result, a security can appear to be overbought or oversold when it is not.
The RSI is a convenient method of labelling a security as being overbought or oversold. However, it is important that the trader remains mindful of market state and inherent volatility facing the product actively being traded. Trending markets, in addition to large periodic spikes in pricing volatility, can limit the capabilities and predictive value.
Ultimately, the RSI is most effective when utilized in concert with other aspects of technical and fundamental analysis.