You need to estimate the mean Christmas sales across a large

You need to estimate the mean Christmas sales across a large chain of jewelry stores. You want to estimate this mean value to within $100 with 99% confidence. Previous studies indicate that the standard deviation for sales across the stores has been $300. How many randomly selected stores from the chain should your sample contain to meet this desired level of accuracy?

Solution

Compute Sample Size
n = (Z a/2 * S.D / ME ) ^2
Z/2 at 0.01% LOS is = 2.576 ( From Standard Normal Table )
Standard Deviation ( S.D) = 300
ME =100
n = ( 2.576*300/100) ^2
= (772.8/100 ) ^2
= 59.722 ~ 60