Standard Deviation in Excel: STDEV.S vs STDEV.P

Standard deviation measures how spread out numbers are from their average. Small = bunched together. Large = scattered.

The formula

=STDEV.S(B2:B100)

STDEV.S or STDEV.P?

• STDEV.S — your data is a sample of a bigger group (50 customers surveyed out of thousands). Use this most of the time.
• STDEV.P — your data is the whole group (every employee in the company).

STDEV and STDEVP are the old names; they still work.

Reading the result

Average 70, SD 8: in roughly bell-shaped data, about two-thirds of values sit between 62 and 78, and about 95% between 54 and 86.

As a percentage of the average

=STDEV.S(B2:B100)/AVERAGE(B2:B100)

Format as %. This is the coefficient of variation — useful for comparing spread across different scales.

With a condition

=STDEV.S(FILTER(C2:C100,A2:A100="North"))

Text and blanks

Both are ignored. STDEVA counts text as 0 — rarely what you want.

Next steps

• Score each value against the spread: Z-score.
• Show it on a chart: error bars.
• Middle value: MEDIAN.