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Just a note

A crucial assumption here is that the sample is random with respect to age. Unfortunately this is often (usually?) not the case.

There is, for example, considerable evidence that trappability of insects varies with age, especially in the first few days of life. Similarly with mammals and birds it is very difficult to get a sample completely unbiased with respect to age, simply because behaviour changes so much in relation to age.

Often, therefore, it is necessary to correct the numbers in the first (and sometimes later) age categories to allow for sampling bias.