Section 9.2.5 contains examples about how many speakers are needed to be 95% certain that the proportion of men and women in the sample differs from that in the population by at most 4%. In this case, the events that were being dealt with (men and women) were equally frequent - p and q were the same at 0.5. If p is quite small as it may be in the case of impostors into speaker verification systems (perhaps less than 1% of the users of a system), it would be necessary to obtain very large samples (running in to tens of thousands). In these cases the Poisson distribution has to be used to estimate sample sizes.