How to calculate spss seasonal index

1. First, judge whether the relationship between the four variables is a multiplication relationship (T*C*S*I) or an addition relationship (T+C+S+I) through the sequence diagram.

2. Secondly, the seasonal variation (S) and irregular variation (I) are eliminated by moving average method or regression equation method, and the long-term trend and periodic variation (T*C or T+C) are calculated.

3. Finally, the seasonal variation and irregular variation components are calculated. The formula of the multiplication model is S*I=Y/T*C, and the addition model is S+I = Y-T-C. The irregular change (I) is eliminated by the average method, and the seasonal index is obtained.