Sample Size for Mean iQ of Statistics Students. The Wechsler IQ test is designed so that the mean is 100 and the standard deviation is 15 for the population of normal adults. Find the sample size necessary to estimate the mean IQ score of Delaware residents. We want to be 95% confident that our sample mean is within 3 IQ points of the true mean. Assume that s = 15 and determine the required sample size.