3.5.4 Comments on Approximations A summary of the approximations discussed above is presented in Figure 3.29. In this figure. H. B. P, and N represent the hypergeometric, binomial, Poisson, and normal distributions. respectively. The widespread availability of modern microcomputers, good statistics software packages, and calculators has made reliance on these approximations largely unnec- essary, but there are situations in which they are useful, particularly in the application of the popular three sigma limit control charts.