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Basic Seasonal Adjustment

Suppose you have monthly retail sales data starting in September, 1978, in a SAS data set named SALES. At this point you do not suspect that any calendar effects are present and there are no prior adjustments that need to be made to the data.

In this simplest case, you need only specify the DATE= variable in the MONTHLY statement, which associates a SAS date value to each observation. To see the results of the seasonal adjustment, you must request table D11, the final seasonally adjusted series, in a TABLES statement.

   data sales;
      input sales @@;
      date = intnx( 'month', '01sep1978'd, _n_-1 );
      format date monyy7.;
      datalines;
   run;
   
   proc x11 data=sales;
      monthly date=date;
      var sales;
      tables d11;
   run;

You can compare the original series, table B1, and the final seasonally adjusted series, table D11 by plotting them together. These tables are requested and named in the OUTPUT statement.

   title 'Monthly Retail Sales Data (in $1000)';
   
   proc x11 data=sales noprint;
      monthly date=date;
      var sales;
      output out=out b1=sales d11=adjusted;
   run;
   
   symbol1 i=join v='star';
   symbol2 i=join v='circle';
   legend1 label=none value=('original' 'adjusted');
   
   proc gplot data=out;
      plot sales    * date = 1
           adjusted * date = 2 / overlay legend=legend1;
   run;

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