The Excel POISSON function calculates the Poisson Probability Mass Function or the Cumulative Poisson Probability Function for a supplied set of parameters.
Syntax: POISSON( x, mean, cumulative )
Where the function arguments are listed in the table below :
|x||–||The number of events that we want the probability for (must be ≥ 0)
This value should be an integer; If a decimal is supplied, it will be truncated to an integer by Excel
|mean||–||The expected number of events (must be ≥ 0)|
|cumulative||–||A logical argument that specifies the type of distribution to be calculated. This can be either:
Poisson Function Examples
|Example 1 – Poisson Probability Mass Function
The chart on the right shows the the Poisson Probability Mass Function with an expected value (mean) of 25.
If you want to use Excel to calculate the value of this function at x = 40, this can be done using the Excel Poisson function, as follows:
=POISSON( 20, 25, FALSE )
This gives the result 0.051917469.
|Poisson Probability Mass Function with mean (λ) = 25|
|Example 2 – Cumulative Poisson Probability Function
The chart on the right shows the the Cumulative Poisson Probability Function with an expected value (mean) of 40.
If you want to use Excel to calculate the value of this function at x = 25, this can be done using the Excel Poisson function, as follows:
=POISSON( 35, 40, TRUE )
This gives the result 0.242414198.
|Cumulative Poisson Probability Function with mean (λ) = 40|
Poisson Function Errors
If you get an error from the Excel Poisson function this is likely to be one of the following :
|#NUM!||–||Occurs if either the supplied x argument or the supplied mean argument is < 0|
|#VALUE!||–||Occurs when either of the supplied x or mean arguments are non-numeric|