To illustrate the method consider the example of a motorway safety officer who believes that the number of accidents per week occurring on a stretch of motorway can be modelled using a Poisson distribution. If X denotes the number of accidents per week then the sample data can be modelled by fitting a Poisson distribution to the sample data. Figure 7.7 provides the tabulated data and the chi-square goodness-of-fit test. The Poisson probability distribution is given by equation (4.10).

Where r = 0, 1, 2, 3 . . . ∞.

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P(X=r) = λ'e-^ r!