What is Standard Deviation?

Standard deviation measures how spread out values are around their mean: a small value means they cluster closely, a large one means they vary a lot.

Standard Deviation explained

It is expressed in the same unit as the data, so a standard deviation of 12 minutes on delivery times means a typical delivery is within roughly that distance of the average.

Databases offer a sample version (STDDEV_SAMP) and a population version (STDDEV_POP); use the sample one when your rows are a sample of something larger.

Example

Two shops both average 100 orders a day, but one varies by about 5 and the other by about 40; the standard deviation shows the second is far less predictable.

Frequently asked questions

What is Standard Deviation?: Standard deviation measures how spread out values are around their mean: a small value means they cluster closely, a large one means they vary a lot.
Can you give an example of Standard Deviation?: Two shops both average 100 orders a day, but one varies by about 5 and the other by about 40; the standard deviation shows the second is far less predictable.

SQL patterns that use it

Outlier Detection in SQL

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