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STDEV.P & STDEV.S

Aggregation

Puts a number on how lumpy revenue is — the difference between a bad month and a signal.

Difficulty

Good

Excel file

1What is it?

A month 15% below average might be a problem or might be Tuesday. Standard deviation tells you which: it measures how far values normally sit from their mean, so a dip inside one standard deviation is ordinary noise and a dip beyond two deserves a question. The choice between the two versions is a real one. STDEV.P treats your data as the whole population; STDEV.S treats it as a sample of something larger and comes out slightly bigger. Twelve months of revenue is a sample of how the business behaves, so STDEV.S is usually right.

2What it looks like

STDEV.S(number1, [number2], …)   ·   STDEV.P(number1, …)

number1, …: The values — a range of months, deals or days. Text and blanks in a range are ignored.

3When you use it

Tell whether this month's dip is noise or a real change.
Set an alert threshold at two standard deviations from the mean.
Compare how volatile two product lines' revenue is.

4See it in action

Change the inputs — the formula and result update live. Prefer the real thing? Download the Excel file and open it in Excel.

Set this month's revenue and see how many standard deviations it sits from normal.

=(B14 - AVERAGE(B2:B13)) / STDEV.S(B2:B13)Twelve-month mean: $41,408

	A
1	Result
2	Twelve-month mean: $41,408
3	STDEV.S: $1,804 — a typical month wanders this far
4	This month is -$5,408 from the mean
5	STDEV.S, not .P: twelve months is a sample of how the business behaves.

The lime cell holds the formula — click it (or any cell) to see its contents in the bar above, just like Excel. Edit the blue cells to watch it recompute.

This month's revenue

5Common errors

#DIV/0!Fewer than two values for STDEV.S, or none for STDEV.P.

Fix: A spread needs at least two points. On a new line, there's nothing to measure yet.

Answer looks too smallSTDEV.P was used on what is really a sample.

Fix: Twelve months is a sample of the business, not the whole population. Use STDEV.S.

Volatility looks enormousThe series is growing, so every month is far from the overall mean.

Fix: Standard deviation measures spread around a FLAT mean. On a trend, look at month-on-month change instead.

Zeros distort itMissing months were entered as 0.

Fix: Leave them blank — zeros count and blanks don't.

6Better functions & alternatives

MEDIAN — Describes the typical value. Standard deviation describes how far from typical things usually wander.
Conditional formatting colour scale — Shows the spread visually without a number, which is often enough for a review. See the tip.

Want STDEV.P & STDEV.S already wired into a model? Wauvel's free tools download as branded, formula-driven Excel.

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