Producing the forecast – season and trend line
Now, we are ready to make the forecast. We are going to multiply the season component by the trend (regression) line to make a prediction for the following years. The concept behind this calculation is that every period of time (in this case, quarters) has an upper or lower inclination given by the seasonal component. The seasonal component moves up or down the trend line, depending on the predictive behavior for this lapse (we're using quarters in this example). The forecast for Year 6 and Year 7 is shown in Figure 12.6:
A forecast is just an approximation of what could happen in the future based on past data. Visualize the sales by quarter for the 5 years of data we have, and note that the sales increase after the third quarter every year, despite sales suffering significant drops in Year 3 and Year 4. This performance is reflected by the forecast...