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# Standard deviation - formula and example

Standard deviation definition. Like Variance, it is a measure of variability of the scores of our sample, how much they differ from the mean. It is variance standardized by taking the square root of variance. We may develop the concept of standard deviation by examining the standard normal curve: Observe that in the standard normal curve there are 3 standard deviations σ on each side of the mean μ, The percentages shown are percentages of the area under the curve. The total area is 100%. Knowing the percentage of area between the mean two point on the standard deviation line allows us to calculate the number of scores contained between these points.

### Standard deviation -formula

The formula for standard deviation is: $$σ =\sqrt{\sum{({X}-{\bar{X})}}^2 \over {n-1}}$$
(Note that σ is the symbol for standard deviation. It is in reality s, that is the square root of variance).
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### What good is sd for?

What good is sd for?

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### Same as variance.

Same as variance.

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