Sample Size for Mean IQ of Statistics Students The Weschler 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 statistics students. We want to be 95% confident that our sample mean is within 2 IQ points of the true mean. The mean for this population is clearly greater than 100. The standard deviation for this population is probably less than 15 because it is a group with less variation than a group randomly selected from the general population; therefore, if we use s 5 15, we are being conservative by using a value that will make the sample size at least as large as necessary. Assume then that s 5 15 and determine the required sample size.