Note that the estimated regression model is Enrollment - 795.5 - 8.6667 * Period + 106.1666 * Qtr_Fall + 58.3333 * Qtr_Winter Thus we get the forecast for each period and then the forecast error for each period. All forecast errors are as follows. 12 2.5 8.5 34 8.5 -103.5 22 39.5 91.5 -68 28.5 20.5 Then we can get the MSE (mean squared error) for this forecast approach, which is the average of the square of the forecast errors