4 pharmaceutical company makes tranquilizers. It is assumed that the distribution for the length of time they last is approximately normal. Researchers in a hospital used the drug on a random sample of nine patients. The effective period of the tranquilizer for each patient (in hours) was as follows: 2.7; 2.8; 3.0; 2.3; 2.3, 2.2; 2.8; 2.1; and 2.4. A 90% confidence interval for the population mean length of time is 2.27, 2.76) degrees of freedom is 7 "90% confident means if we were to sample many groups of nine patients. 90% of the samples would contain the true population mean length of time. sample standard deviation s = 2.51 We need to use a Student's-t distribution, because we do not know the population mean