1) The moving average for x periods is calculated,
2) The average of two consecutive moving averages is calculated in order to centre the moving average for one given period. This step is necessary since the number of periods is even (e.g., 4
seasons, 12 months, etc.),
3) The ratio-to-moving-average is calculated by dividing the orig-
inal percentage by the centred moving average,
4) The seasonal index is then calculated by averaging and
normalizing the ratios for each period.