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 Y and advertising as X and think of the data as 36 weeks of information.)