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Don Miller and Dan Williams have recently re-posted spreadsheets and X-12 specifications that can be used to implement the seasonal damping method they proposed in "Shrinkage Estimators Of Time Series Seasonal Factors And Their Effect On Forecasting Accuracy," International Journal of Forecasting 19(4): 669-684, and "Damping seasonal factors: Shrinkage estimators for the X-12-ARIMA program," International Journal of Forecasting 20(4): 529-549.

Damping is needed to counteract excessive seasonal variation produced by classical decomposition and X-12, which is an artifact of the random noise in the data.These articles show that a small but significant reduction in forecast error is obtained when using seasonal damping. The benefit is more pronounced when seasonal factors are determined through Classical Decomposition, but remains significant when X-12 is used.

The spreadsheet software for seasonal factor damping is available from the Software Page.