The Poisson distribution is a discrete probability function that is used to calculate the probability of a number of events occurring in a specified time period.
where λ is the expected number of occurrences within the specified time period.Further information on the Poisson Distribution is provided on the Wikipedia Poisson Distribution Page
The Excel POISSON.DIST function calculates the Poisson Probability Mass Function or the Cumulative Poisson Probability Function for a supplied set of parameters.
The function is new to Excel 2010 and so is not available in earlier versions of Excel. However the Poisson.Dist function is simply a new version of the Poisson function, which is available in earlier versions of Excel.
The format of the function is:
Where the function arguments are:
|x||-||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).|
The above chart on the right shows the Poisson Probability Mass Function with an expected value (mean) equal to 25.
If you want to calculate the value of this function at x = 40, this can be done using the Excel Poisson.Dist function, as follows:
This gives the result 0.051917469.
The above chart on the right shows the Cumulative Poisson Probability Function with an expected value (mean) equal to 40.
If you want to calculate the value of this function at x = 25, this can be done using the Excel Poisson.Dist function, as follows:
This gives the result 0.242414198.
Further information and examples of the Excel Poisson.Dist function can be found on the Microsoft Office website.
If you get an error from the Excel Poisson.Dist function this is likely to be one of the following:
|#NUM!||-||Occurs if either the supplied value of x, or the supplied mean is < 0.|
|#VALUE!||-||Occurs if any of the supplied arguments are non-numeric.|