for the population of people who suffer from insomnia, suppose p= 0.6 is the proportion that get some relief from taking lunesta. for a random sample of 48 people who suffer from insomnia, give the mean and the standard error of the sampling distribution of the sample proportion. Solution we know, mean = np , where , n = no of people, p = prob of success (in this case =0.6) so, mean = 48 x 0.6 = 28.8 now, std dev = np(1-p) = 48 x 0.6 x 0.4 = 11.52.