Suppose you want to construct a 95% confidence interval for the mean age that people have their wisdom teeth removed.  If you know that the standard deviation is 1.8 and you want a margin of error of no more than plus or minus 0.2 years, at least how many people must you survey?


Solution

We use the formula:

n=(z sigma / E)^2

Now write down the cast of characters:

s = 1.8

E = 0.2

c = 0.95

z = 1.96

Plug in to get

n=(1.96*1.8/0.2)^2 = 311.1696

Next note that we can't survey a fractional number of people.  For sample size calculations we always round up.  We can conclude that in order to construct this 95% confidence interval with a margin of error of no more than plus or minus 0.2 years we need to survey at least 312 people.

Back to the handout

This site has had Hit Counter visitors since February 26, 2001