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...

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 of 30 stores this month shows mean sales of 83 units of a small appliance, with a standard deviation of 12.2 units. During the same month last year, an SRS of 28 stores have mean sales of 97.274 units, with standard deviation 4.4 units. An increase from 83 to97.274 is a rise of about 17%.


1. Construct a 95% confidence interval estimate of the difference μ1−μ2, where μ1 is the mean of this year's sales and μ2is the mean of last year's sales.

(a)  <>

(b) The margin of error is