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← The library

Statistical function

POISSON.DIST

Returns the Poisson distribution probability.

Rarely usedDifficulty 1450 · AdvancedUsage rank #381 of 520

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When to use it

The Poisson distribution: the probability of a count of events in an interval when they occur at a known average rate. Calls per hour, defects per batch, arrivals per minute.

The shape of it

Syntax: =POISSON.DIST(x, mean, cumulative)

Worked examples

Exactly two
=POISSON.DIST(2,3,FALSE) → 0.224041808
Exactly 2 events when the average is 3.
Two or fewer
=POISSON.DIST(2,3,TRUE) → 0.42319008
Two or fewer.
Zero events
=POISSON.DIST(0,3,FALSE) → 0.049787068
No events at all: e to the minus 3.

Worth knowing

Staffing: 1 - POISSON.DIST(capacity, expected arrivals, TRUE) is the chance of being overwhelmed.
Mean equals variance in a Poisson process; if your data is much more variable, use NEGBINOM.DIST.
Rate must be per the same interval as the count.

Where it goes wrong

#NUM! for a negative mean or count.
Non-integer counts are truncated.

AVERAGE COUNT COUNTA COUNTIF COUNTIFS MAX

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