A random sample of 90 observations produced a mean A 25.9 and a standard deviation s = 2.7. find a 95% confidence interval for the population mean mu. find a 90% confidence interval for mu. Find a 99% confidence interval for mu. Solution a. 95 % confidence interval = +- Z Z= 1.96 25.9 + -1.96 * 2.7 (31.192, 20.608) b. Z= 1.645 25.9 +- 1.645 * 2.7 (21.4585 , 30.3415) c. Z= 2.576 25.9 +- 2.576*2.7 (18.9448 , 32.8552).