A medical statistician wants to estimate the average weight

A medical statistician wants to estimate the average weight loss of people who are on a new diet plan. In a preliminary study, he guesses that the standard deviation of the population of weight loss is about 11 pounds. How large a sample should he take to estimate the mean weight loss to within 5 pounds, with 91% confidence? Sample Size =

Solution

The z-value from normal probability table with 91% confidence level is 1.666

n= [ z sd /error]^2
n= (1.666)^2(11)^2 /5^2

n=13.4 = 13 samples