A study was conducted in order to estimate the mean number

A study was conducted in order to estimate ?, the mean number of weekly hours that U.S. adults use computers at home. Suppose a random sample of 81 U.S. adults gives a mean weekly computer usage time of 8.5 hours and that from prior studies, the population standard deviation is assumed to be ? = 3.6 hours.

How large a sample of U.S. adults is needed in order to estimate ? with a 95% confidence interval of length 1.2 hours?

A 6

B 12

C 20

D 36

E 144

Solution

Compute Sample Size
n = (Z a/2 * S.D / ME ) ^2
Z?/2 at 0.05% LOS is = 1.96 ( From Standard Normal Table )
Standard Deviation ( S.D) = 3.6
ME =1.2
n = ( 1.96*3.6/1.2) ^2
   = (7.056/1.2 ) ^2
   = 34.574 ~ 35