Читать книгу Medical Statistics - David Machin - Страница 117

Suppose events happen randomly and independently in time at a constant rate. If the events happen with a rate of λ events per unit time, the probability of r events happening in unit time is

(4.2)

where exp(−λ) is a convenient way of writing the exponential constant e raised to the power −λ. The constant e being the base of natural logarithms which is 2.718281…

The mean of the Poisson distribution for the number of events per unit time is simply the rate, λ. The variance of the Poisson distribution is also equal to λ, and so the SD = √λ.