Elementary Statistics: Picturing the World (6th Edition)

If np $\geq$ 5 and nq $\geq$ 5 then the normal distribution can be used to approximate the binomial. np = 18 $\times$ 0.90 = 16.2 nq = 18 $\times$ 0.10 = 1.8 Since np $\geq$ 5 but nq $\lt$ 5, we cannot use the normal distribution to approximate the distribution of x.