What is standard deviation in trading?
In one line: Standard deviation measures how spread out a set of numbers is around its average. A bigger number means results swing more; a smaller one means they're steadier.
The idea
The average tells you the middle. Standard deviation tells you how far results usually land from that middle.
Two traders can have the same average and very different rides.
A simple example, step by step
Two sets of 4 trade results, in units of risk (R):
- Set A: +2, 0, +2, 0
- Set B: +4, −2, +4, −2
Step 1. Find the average. A: (2 + 0 + 2 + 0) ÷ 4 = 1R. B: (4 − 2 + 4 − 2) ÷ 4 = 1R. Same average.
Step 2. Find each result's distance from the average. A: +1, −1, +1, −1. B: +3, −3, +3, −3.
Step 3. Square those distances. A: 1, 1, 1, 1. B: 9, 9, 9, 9.
Step 4. Average the squares. A: 1. B: 9.
Step 5. Take the square root. A: 1R. B: 3R.
Same average, but Set B swings 3 times as much. A math example, not a promise.
(Many tools divide by one less than the count in step 4, which makes the number a bit bigger for small sets. The idea is the same.)
Why it matters
Standard deviation shows up all over trading:
- Your results. A big spread means deeper dips along the way, even with the same average. See max drawdown.
- Price moves. It's one way to measure Volatility. Bollinger Bands are drawn using it.
- Ratios. The Sharpe ratio divides return by standard deviation. The Sortino ratio uses only the downside part.
A common mistake
Thinking a bigger number is bad by itself. Standard deviation counts big wins as "spread" too. A method with rare large wins will have a high one. Look at it alongside the average and the worst dips, not alone. See mean and median.
For price swings in points, ATR is often easier to use. Compare them in ATR vs standard deviation.
Questions
What does a high standard deviation mean in trading?
Results or prices swing widely around their average. That can mean bigger dips along the way, even if the average is fine.
How is standard deviation used in trading?
To measure volatility, to draw Bollinger Bands, and inside ratios like Sharpe that compare return with how bumpy the ride was.
Is standard deviation the same as ATR?
No. Standard deviation measures spread around an average. ATR measures the typical size of each candle's range.
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