One recurring theme in our forthcoming book, Modelling Mortality with Actuarial Applications, is the all-pervading role of likelihoods that suggest the lurking presence of a Poisson distribution.  A popular assumption in modelling hazard rates is that the number of deaths observed at any given age is a Poisson random variable, so perhaps that might explain it?  Surprisingly, it is the other way round — it is the very nature of the data in a survival model that leads inexorably to the Poisson distribution, even if we assume no such thing.

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Want to find out more about the book? Take a look at our website and co-author Iain Currie’s post Mortality By The Book