The input data set must contain either quarterly or monthly time series, and the data must be sorted in chronological order within each BY group. For the standard X-12-ARIMA method, there must be at least three years of observations (12 for quarterly time series or 36 for monthly).
If an ARIMA model is specified in the ARIMA statement, AUTOMDL statement, PICKMDL statement, or the MDLINFOIN= data set, then more than three years of observations might be required in order to fit the ARIMA model and perform the computations associated with the seasonal decomposition and other diagnostics.
The minimum number of observations applies to each series listed in the VAR statement and within each BY group and is determined after any missing values are trimmed from the series.