Published 19 November 2008, doi:10.1136/bmj.a2600
Cite this as: BMJ 2008;337:a2600

Endgames

Statistical question

Distributions

John Fletcher, clinical epidemiologist

1 BMJ, London WC1H 9JR

jfletcher{at}bmj.com

The average number of suicides reported each month in one county last year was 3.2. To compare numbers of suicides between months, calculating P values or confidence intervals, which probability distribution would be most appropriate to use?

a) Binomial
b) Normal
c) Poisson
d) Weibull

Show Answers

Answer

C—the Poisson distribution describes the occurrence of rare events happening at random in a large population, which fits the bill here. It is also necessary to check that there are not linkages between the suicides, or clustering, as this will affect the interpretation of the expected probabilities. The Poisson distribution is helpful for "back of the envelope" calculations because, when the number of events is reasonably large, the standard deviation of the distribution is the same number as the mean. (Technically the binomial distribution could be used but it requires long calculations of large numbers.)

Cite this as: BMJ 2008;337:a2600


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