What is the relative strength index (RSI)?

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The technical trader’s “sign” wave.

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When a stock is rallying, we assume there’s an influx of buyers willing to pay higher prices for the shares. Similarly, when a stock’s price is falling, we assume the opposite, that sellers are trying to unload assets onto buyers who are willing to pay only lower prices. But amid the bidding-up or offering-down process, might there be a way to gauge whether a stock might be “overbought” and ripe for a pullback, or “oversold” and poised for a bounce?

In terms of big-picture fundamentals, there is no tool designed to measure the likelihood of a short-term reaction, whether a pullback or bounce. But on a technical level, there is a momentum oscillator designed to do just that. It’s called the relative strength index, or RSI for short, and its purpose is to measure the speed and size of a stock’s price movements, giving investors a clue as to when a price swing might be set for a break before resuming or reversing its original direction.

The relative strength index is a momentum oscillator that aims to measure the speed and size of an asset’s price movements in order to evaluate whether the asset may be in “overbought” or “oversold” conditions. It was developed by J. Welles Wilder and introduced in his book New Concepts in Technical Trading Systems in 1978.

In simple terms, the RSI aims to indicate when a stock’s price may be too high and poised for a pullback, or too low and poised for a bounce. It’s a tool that may help you anticipate market reactions. But note that the duration or scale of the reaction isn’t something the RSI was designed to predict. Not every correction will be a reversal. And even if a reversal does occur, it won’t necessarily happen in short order.

The RSI displays with a value range of 0 to 100. There are two prominent lines on the oscillator chart: a lower line at 30 and a higher line at 70. In theory, the area below 30 is considered oversold, while the area above 70 is considered overbought. See figure 1 for an example.

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Figure 1: HOLD YOUR HORSES, THERE! Sometimes a market gets ahead of itself—overly exuberant on the highs and overly discouraging on the lows. RSI signals when a market may be overbought or oversold. For educational purposes only.

Source: Barchart.com. Annotations by Encyclopædia Britannica.

It’s important to remember that these overbought and oversold levels are theoretical. They provide a reference point for possible reactions against the trend (aka corrections). Whether a stock is truly overbought or oversold on the basis of real demand is beyond the oscillator’s capacity to determine. In other words, don’t take these technical indicators as a reflection of fundamental conditions.

So, how might you use the RSI to enhance your trading strategy? Here are a few suggestions from Wilder’s book.

Anticipate tops and bottoms. Potential market tops can occur when the RSI goes above 70; relative bottoms, when RSI moves below 30. These are relative tops and bottoms that may eventually continue in their original direction or reverse, depending on the fundamental conditions of the stock or the broader market.

Chart patterns on the RSI. There are chart patterns that may show up in the RSI but not on the actual bar chart. For instance, bottoming and topping formations may contradict the price action. This is known as divergence, and it’s a critical indication of a potential trend reversal (see below).

Divergence. When the RSI reverses upward from below the 30 range while the price trend continues to swing lower, that’s considered a bullish divergence, and a sign that a stock may be bottoming and poised to bounce. When the RSI falls from above the 70 range while the price trend continues to swing higher, that’s considered a bearish divergence, signaling that a top may be in place and that prices are likely to pull back.

Failure swings. In an uptrend, prices must make higher highs and higher lows. If a price swing fails to make a higher high while the RSI scale is above 70, it’s a strong indication that prices are likely to reverse toward the downside.

In contrast, a downtrend must continue to make lower lows and lower highs. If prices fail to break below their previous lows while RSI is below 30, it’s an indication that prices may soon reverse toward the upside.

Support and resistance. Support and resistance can sometimes show up on the RSI before the price chart, giving you a heads-up that prices may soon reverse course.

The RSI was designed to help traders identify potential reactions or reversals from overbought and oversold levels. Although it’s a popular and widely used tool, you should be aware of its limitations.

The RSI can indicate many different but related outcomes. Try combining it with other technical and fundamental indicators that can help provide you with a bigger picture of the surrounding market dynamics.

New Concepts in Technical Trading Systems, J. Welles Wilder, Jr. Hunter Publishing Co., 1978.