35 Appliance sales A research firm supplies manufactur ers with estimates of the sales of their products from samples of stores Marketing managers often look at the sales estimates and ignore sampling error An SRS of 50 stores this month shows mean sales of 41 units of a particular appliance with standard deviation of 11 units During the same month last year an SRS of 52 stores gave mean sales of 38 units of the same appli ance with a standard deviation of 13 units An increase from 38 to 41 is a rise of 7 9 The marketing manager is happy because sales are up 7 9 a Give a 95 confidence interval for the difference in mean number of units of the appliance sold at all retail stores b Explain in language that the manager can under stand why he cannot be confident that sales rose by