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.