Since the binomial distribution is discrete and the normal distribution is continuous, it is com- mon practice to use continuiry corrections in the approximation, so that where 4 denotes the standard normal cumulative distribution function. Other types of prob- ability statements are evaluated similarly. such as The normal approximation to the binomial is known to be satisfactory for p of approximately I/2 and n 10. For other values of p, larger values of n are required. In general. the approxi- mation is not adequate for p l/(n 1) or p n/(n t l). or for values of the random vari- able outside an interval six standard deviations wide centered about the mean e the interval np t 3 (np (1 -p)). We may also use the normal approximation for the random variable p rhn- that is, the sample fraction defective of Section 3.2.2. The random variable p is approximately nor- rnally distributed with mean p and variance p(1 p)ns so that Plus p s a 4 p) n Since the normal will serve as an approximation to the binomia and since the binomial and Poisson distributions are closely comnected, it seems logical that the normal may serve to approximate the Poisson. This is indeed the case, and if the mean of the Poisson distribu tion is large say, at least 15-then the normal distribution with u and G is a satisfactory approximation.