Syntax
POISSON(x, mean, cumulative)
| Parameter | Description |
|---|---|
| x | Parameter of the POISSON function. |
| mean | Parameter of the POISSON function. |
| cumulative | Parameter of the POISSON function. |
Examples
Exactly 10 emails in an hour
Formula
=POISSON(10, 8, FALSE)
Returns ~0.0993. With an average of 8 emails/hour, there is about a 9.9% chance of receiving exactly 10.
5 or fewer cars passing in 10 min
Formula
=POISSON(5, 7, TRUE)
Returns ~0.3007. If 7 cars pass on average every 10 minutes, there is about a 30% chance of 5 or fewer passing.
Probability of at least one error
Formula
=1-POISSON(0, 0.5, FALSE)
Returns ~0.3935. With 0.5 expected errors per page, the probability of at least one error is about 39.4%.
Common Errors
#NUM!
x must be non-negative and mean must be non-negative.
#VALUE!
Occurs when arguments are non-numeric text.
Tips
Use POISSON.DIST instead
POISSON is the compatibility version. POISSON.DIST is the modern replacement with identical functionality.
Good approximation for rare events
The Poisson distribution works best for rare events over many trials. If the event probability is very small and the number of trials is large, Poisson approximates the binomial well.
Mean equals variance
A unique property of the Poisson distribution: the mean and variance are equal. If your data's variance differs significantly from its mean, Poisson may not be the right model.
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