A science fiction enthusiast owns hundreds of books and wants to know how many pages the books contain, on average. A random sample of 15 books has an average of 216 pages and a standard deviation of 24 pages. To calculate a confidence interval for the population mean, what value would we use for the degrees of freedom? A science fiction enthusiast owns hundreds of books and wants to know how many pages the books contain, on average. A random sample of 15 books has an average of 216 pages and a standard deviation of 24 pages. To calculate a confidence interval for the population mean, what value would we use for the degrees of freedom