RSI：含义、示例与风险

The Relative Strength Index (RSI) is a momentum oscillator that measures the speed and change of price movements on a scale from 0 to 100. It compares average gains with average losses over a selected lookback period, commonly 14 periods. Readings above 70 are often described as overbought, while readings below 30 are often described as oversold. These levels identify momentum conditions, not guaranteed reversals or trading outcomes.

Developed by J. Welles Wilder in 1978, RSI converts recent upward and downward price changes into a normalized momentum reading. Because it uses historical prices, it is backward-looking and may respond after a move has begun. Analysts commonly review its level, changes over time, threshold crossings, and divergences from price. RSI can remain at an extreme during a strong trend, so a high or low reading should not automatically be interpreted as a direction to buy or sell.

What RSI means

RSI expresses the relationship between average gains and average losses. Its formula is RSI = 100 - [100 / (1 + RS)], where RS equals average gain divided by average loss over the selected number of periods. A high value means recent gains have been large relative to recent losses; a low value means losses have dominated. The conventional 70 and 30 thresholds are interpretive reference points. Above 70 may indicate an overbought or extended condition, and below 30 may indicate an oversold condition, but neither establishes that a reversal is imminent. Analysts also examine divergence. For example, if price reaches a new high while RSI remains below its prior high, momentum may be weakening. Divergence is only a warning condition and may occur without a reversal.

工作原理

For each period, calculate the change from the previous closing price. Record a positive change as a gain and a negative change by its absolute value as a loss; the other side is recorded as zero. Calculate average gains and average losses for the initial lookback window, commonly 14 periods. Divide average gain by average loss to obtain RS, then insert RS into the RSI formula. Wilder’s method initializes the calculation with simple averages and then applies recursive smoothing to subsequent gains and losses. This helps update RSI as each new period is added. Shorter lookbacks make the oscillator more responsive but noisier, while longer lookbacks produce a smoother, less responsive reading. Charting implementations may differ, so the lookback and averaging method should be checked when comparing values.

Practical example

Assume a hypothetical asset closes at $10, $11, $12, $11, $12, and $13. The five period-to-period changes are +$1, +$1, -$1, +$1, and +$1. For the gain series, the decline is recorded as zero, producing gains of 1, 1, 0, 1, and 1. For the loss series, only the decline is recorded by its absolute value, producing losses of 0, 0, 1, 0, and 0. The initial average gain is 4/5, or 0.8, and the initial average loss is 1/5, or 0.2. RS is 0.8 divided by 0.2, which equals 4. RSI is therefore 100 - [100 / (1 + 4)] = 80. Under conventional thresholds, 80 would be described as overbought. It does not show that the price must fall; it only indicates that gains dominated losses in this simplified window.

Risks and limitations

RSI can produce misleading or late signals because it is calculated from past prices. In a strong uptrend, it may remain above 70 for an extended period, while in a strong downtrend it may remain below 30. Treating either threshold as an automatic reversal signal can therefore conflict with the prevailing trend. Divergences can also persist or fail to lead to a reversal. A shorter lookback may create more frequent fluctuations, whereas a longer one may react more slowly. Results can vary with the chosen timeframe, lookback, data, and smoothing implementation. RSI measures price momentum only; it does not guarantee performance or remove the risk of loss.

Frequently asked questions

What is the common RSI lookback period?

A 14-period lookback is common. A period can represent a day, hour, week, or another chart interval. Shorter settings are more responsive and noisier; longer settings are smoother and slower.

What do RSI readings above 70 or below 30 mean?

Above 70 is commonly labeled overbought and below 30 oversold. These labels describe relative momentum, not certain price reversals.

What is RSI divergence?

Divergence occurs when price and RSI do not confirm one another. A new price high accompanied by a lower RSI high may indicate weakening upward momentum, but it does not guarantee a decline.

Does RSI predict future prices?

No. RSI is based on historical price changes. It can highlight momentum conditions and possible warning signs, but it cannot reliably forecast a specific future price or outcome.

Educational note: This material explains a market concept; it is not personalized investment advice.

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