Out of Sample Testing / Holdout Sample
Out-of-sample testing is a popular way to test the likely accuracy of a forecasting method. It is unbiased since it’s stripped of all adjustments and filters.
To conduct this test, take out the most recent periods of demand history (the holdout sample) as if it did not exist. The number of time periods that you remove should correlate to your normal forecast horizon (e.g. if you forecast three months into the future, the holdout should be at least three months long). You can then apply different forecasting methods to see what the holdout sample error is or Mean Absolute Deviation/Error (MAD or MAE). This will tend to be higher than the mean error. The method with the lowest MAD will likely be more accurate.
The holdout sample is strictly anecdotal data since it covers only a limited number of periods. So while it’s helpful to test different methods, it does not, by itself, determine which method is best.