Question

A department store serving a community wants to estimate customer support in the morning hours. From past studies, the standard deviation is known to be estimated at 45 clients.

a) What size should the sample be if you want to be 90% confident that the correct period varies between +/- 5 clients.

Answer 1

Solution :

Given that,

standard deviation =   =45

Margin of error = E = +/-5

At 90% confidence level

= 1 - 90%  

= 1 - 0.90 =0.10

/2 = 0.05

Z/2 = Z _{0.05 = 1.645}

sample size = n = [Z/2 * / E] ²

n = ( 1.645 * 45 / 5 )²

n =219.1880

Sample size = n =220