Technical analysis

Mean reversion

Mean reversion is the tendency of a variable, such as a stock’s price or return, to move back toward a specified historical average after deviating from it.

What it means

A stock can move away from its recent average and later move closer to it. That pattern is called mean reversion, but it is a hypothesis to test, not a rule that every price follows. The relevant “mean” might be an average price, return or valuation measure over a defined period. Changing the measure or period changes the reference point.

A price below its average is not necessarily undervalued. New information can change what investors are willing to pay, and a price can remain below its old average—or move further away—for a long time. Mean reversion contrasts with momentum, which describes a tendency for a movement to continue rather than reverse.

How it is calculated

There is no single mean-reversion formula. A simple way to describe how far a stock’s closing price is from its recent average is:

Deviation (%) = (P_t − MA_n,t) / MA_n,t × 100

  • P_t is the stock’s closing price on day t, in US$ per share.
  • MA_n,t is the arithmetic mean of the closing prices for the n trading days ending on day t, in US$ per share.
  • n is the number of trading days in the averaging window.
  • Deviation (%) is the percentage difference between the closing price and that average. A negative value means the close is below the average.

This measures a deviation; it does not establish that a reversal will happen. Testing whether a series tends to revert requires examining repeated observations over a stated horizon and accounting for periods when the pattern fails. For stock prices, changes in trend can make a past average a poor reference.

Worked example

Suppose a hypothetical stock has closed at US$100, US$100, US$100 and US$100 over four trading days, then closes at US$90 on the fifth day. Use a five-day moving average that includes today’s close.

  1. Add the five closing prices: US$100 + US$100 + US$100 + US$100 + US$90 = US$490 per share.
  2. Divide by five: MA_5,t = US$490 / 5 = US$98 per share.
  3. Calculate the deviation: (US$90 − US$98) / US$98 × 100 ≈ −8.16%.

The close is about 8.16% below its five-day average. That is a description of past prices, not an estimated probability of recovery. The average already includes the US$90 close; using the previous day’s five-day average would produce a different result.

Why it matters before you trade

A mean-reversion idea depends on both the direction and timing of a potential move. Before placing an order, a private investor can distinguish the proposed entry rule from the evidence that the pattern has held across comparable periods. A sharp move may reflect earnings, a trading halt or another event rather than a temporary deviation.

Execution also matters. The bid-ask spread, market impact and other transaction costs can consume a small move toward an average. Volatility affects how far a price might move in either direction before any reversion occurs. A pre-trade check therefore addresses risks that a price chart alone cannot settle; it cannot make the pattern reliable.

How PreTrAIde uses it

PreTrAIde does not claim to identify or trade mean-reversion signals. Its pre-trade analysis is different from chart-pattern technical analysis. A user builds a stock basket with a side, quantity and execution conditions for each line, then runs an analysis before placing any order. PreTrAIde returns GO, NO-GO or MODIFY verdicts for the basket and each line, alongside model-estimated success probabilities, risk assessments, suggested execution strategies and plain-language reasoning.

Its quantitative checks include estimated spread, participation in daily volume, market impact, daily volatility and one-day 95% value at risk. These can provide execution and risk context for a proposed trade based on mean reversion, but they do not validate that the stock will return to an average. The verdicts are decision support, not recommendations; users place any trades themselves through their own broker.

Common mistakes

  • Treating the moving average as a fixed fair value rather than a changing summary of past prices.
  • Assuming a large negative deviation must be followed by a gain, or that a positive deviation must reverse.
  • Switching averaging windows after seeing the result without recognizing how that changes the signal.
  • Ignoring events, spreads, market impact or the time a position might remain away from its average.
  • Mistaking a backtested pattern for a guarantee; a rule fitted closely to past data may not hold in new data.

Frequently asked questions

Does mean reversion mean a stock price will return to its moving average?

No. A moving average summarizes past prices, while news and changing market conditions can shift the price for an extended period. A deviation alone says nothing certain about the direction or timing of the next move.

How is mean reversion different from momentum?

Mean reversion concerns movement back toward a specified average after a deviation. Momentum concerns the continuation of an existing movement; neither pattern applies reliably to every stock or period.

Can a stock be below its average without being undervalued?

Yes. A historical price average is not an estimate of intrinsic value. The stock may have fallen because new information changed expectations about the company.

Related terms

Moving average Moving average is the average of a stock’s prices over a rolling number of trading periods, used to smooth short-term fluctuations and describe its past price trend. Momentum Momentum is the tendency of a stock’s recent price direction to persist for a period, often measured by its return over a specified lookback window. Volatility Volatility is a measure of how widely an asset’s returns vary over a specified period, usually expressed as the standard deviation of returns. Transaction costs Transaction costs are the explicit fees and implicit trading costs incurred when executing a trade, including commissions, the bid-ask spread and market impact. Pre-trade analysis Pre-trade analysis is the assessment of a proposed stock order or basket before placement, estimating execution costs, liquidity and risk to inform the investor’s decision and execution approach. Overfitting Overfitting is when a model learns quirks of its training data rather than patterns that generalize, making its apparent performance unreliable on new data.

Technical analysis

Technical analysis Relative strength index (RSI) MACD Bollinger Bands Support and resistance Trend following Breakout Market sentiment analysis All glossary terms →

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Educational content, not investment advice. PreTrAIde is analysis software, not a broker: it never executes orders, and its verdicts are decision support, not recommendations to buy or sell.