Modifying Predictor Values in the Forecast Period

Testing the two scenarios of mailing more catalogs or providing more phone lines requires modifying the estimates for the predictors mail or phone, respectively. Because we're only modifying the predictor values for three cases (months), it would be easy to enter the new values directly into the appropriate cells of the Data Editor. For instructional purposes, we'll use the Compute Variable dialog box. When you have more than a few values to modify, you'll probably find the Compute Variable dialog box more convenient.

  1. From the menus choose:

    Transform > Compute Variable...

    Figure 1. Compute Variable dialog box
    Compute Variable dialog box
  2. Enter mail for the target variable.
  3. In the Numeric Expression text box, enter mail + 2000.
  4. Click If.
    Figure 2. Compute Variable If Cases dialog box
    Compute Variable If Cases dialog box
  5. Select Include if case satisfies condition.
  6. In the text box, enter $CASENUM > 120.

    This will limit changes to the variable mail to the cases in the forecast period.

  7. Click Continue.
  8. Click OK in the Compute Variable dialog box, and click OK when asked whether you want to change the existing variable.

This results in increasing the values for mail--the number of catalogs mailed--by 2000 for each of the three months in the forecast period. You've now prepared the data to test the first scenario, and you are ready to run the analysis.

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