A company tracks the level of sales at retail outlets weekly for 36 weeks. During the first 12 weeks, a fixed level of advertising was used each week to draw in customers. During the second 12 weeks,...

A company tracks the level of sales at retail outlets weekly for 36 weeks. During the first 12 weeks, a fixed level of advertising was used each week to draw in customers. During the second 12 weeks, the level of advertising changed. During the last 12 weeks, a third level of advertising was used. What does the SRM have to say about the average level of sales during these three periods? (Treat sales as
 and advertising as
 and think of the data as 36 weeks of information.)