I hope your MBA students are listening to you, Steve. Before I got into quality, I did budgeting and strategic planning that involved a lot of forecasting. Since I had an MBA, I used many of the techniques that are in your chapter. Even worse, I sometimes reset my "target" forecast based on the actual versus forecast. My forecasts started showing more and more variation from actual when I made this adjustment. If Steve has not shown you the funnel experiment yet, it will explain what happened to my forecasts.
I finally realized that by forecasting the average with some adjustments for known variation (e.g. new customers), I had better forecasts. Later when I learned SPC, I realized I should have done a control chart and used the average as my forecast with the control limits showing the most likely highest and lowest forecast. For teaching purposes, text books normally show examples where there is a steady increase or decrease in the data. Unfortunately, much of the business world data that the students will see is more like Steve's example.
Bill Pflanz