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.