A market research firm supplies manufacturers with estimates of the retail sales of their products from samples of retail stores. Marketing managers are prone to look at the estimate and ignore sampling error. An SRS of28

28

stores this year shows mean sales of77

77

units of a small appliance, with a standard deviation of9.8

9.8

units. During the same point in time last year, an SRS of17

17

stores had mean sales of68.278

68.278

units, with standard deviation10.7

10.7

units. An increase from68.278

68.278

to77

77

is a rise of about 11%.

1. Construct a 95% confidence interval estimate of the difference

1

2

1

2

, where

1

1

is the mean of this year's sales and

2

2

is the mean of last year's sales.