An IQ test is given to a randomly selected group of 10 freshmen at a given university and also to a randomly selected group of 5 seniors at the same university. For the freshmen, the sample mean is 120 and the (unbiased)

sample variance is 196. For the seniors, the sample mean is 128 and the

(unbiased) sample variance is 121. Determine a 95 percent confidence interval for ps — pf, where ps is the population mean IQ for the freshmen and pf is the population mean IQ for the seniors (assume that the population variances are equal for the two populations).