Confidence interval for the population standard deviation A psychologist wants to estimate the standard deviation of IQ scores. It is widely believed that IQ scores follow a normal distribution. Her random sample of 25 IQ scores has a mean of 98.8 and a standard deviation of 17.1 . Find the 90% confidence interval for the population standard deviation based on this sample. Then give its lower limit and upper limit. Carry your intermediate computations to at least three decimal places. Round your answers to at least two decimal places. (If necessary, consult a list of formulas.) Lower limit: X 5 ? Upper limit