Suppose now that a strike at ZXY Company during July and August of 1988 caused sales to decrease an estimated 50%. Since this is a one-time event with a known cause, it is appropriate to prior adjust the data to reflect the effects of the strike. This is done in PROC X11 through the use of PMFACTOR=varname (prior monthly factor) in the MONTHLY statement.

In the following example, the PMFACTOR variable is named PMF. Since the estimate of the decrease in sales is 50%, PMF has a value of 50.0 for the observations corresponding to July and August 1988, and a value of 100.0 for the remaining observations.

This prior adjustment on SALES is performed by replacing SALES with the calculated value (SALES/PMF) * 100.0. A value of 100.0 for PMF leaves SALES unchanged, while a value of 50.0 for PMF doubles SALES. This value is the estimate of what SALES would have been without the strike. The following example shows how this prior adjustment is accomplished.

   data sales2;
      set sales;
      if '01jul1988'd <= date <= '01aug1988'd then pmf = 50;
      else pmf = 100;
   run;
   
   proc x11 data=sales2;
      monthly date=date pmfactor=pmf;
      var sales;
      tables a1 a2 a3 d11;
      output out=x11out a1=a1 a2=a2 a3=a3 d11=d11;
   run;

Table A2 contains the prior monthly factors (the values of PMF), and Table A3 contains the prior adjusted series.