Question

1. You no longer believe that the population standard deviation in the yearly sales per store of this model of camera is a known quantity. You therefore will use the sample standard deviation of the yearly sales per store as an estimate of this unknown population standard deviation as you attempt to estimate the yearly mean sales per store of the population of stores across the United States. You collect sales information from a random sample of stores. This data is shown in appendix one below. Find both 95% and 90% confidence intervals for the mean sales per store at this population of stores. At each confidence level, comment upon the change in the results of this problem from the results of the previous problem.

            Appendix One (Sales per store of the unit)

            143     189     201     134     155     198     225     235     188     165     141

            199     154     219     215     167     187     219     220     168     155     189

            230     214     145     139     187     190     176     190

Answer 1

Find sample size , mean and standard deviation of the samples

Formula used to calculate the mean, standard deviation and sample size in excel:

90% Confidence Interval: 175.58 < μ < 193.56

(Refer column F for the calculation procedures and formulas used)

95% confidence interval: 173.85 < μ < 195.28

With a 95 percent confidence interval, you have a 5 percent chance of being wrong. With 90 percent confidence interval, you have a 10 percent chance of being wrong

Limits of the Confidence interval 95% is wider than the 90% confidence interval.