Modeling the Recurrence Probability by Period

A problem with the model as it stands is that it ignores the information gathered at the first examination; that is, that many patients did not experience a recurrence in the first six months. A "better" model would model a binary response that records whether or not the event occurred during each interval. Fitting this model requires a reconstruction of the original dataset, which can be found in ulcer_recurrence_recoded.sav. See the topic Sample Files for more information. This file contains two additional variables:

  • Period, which records whether the case corresponds to the first examination period or the second.
  • Result by period, which records whether there was a recurrence for the given patient during the given period.

Each original case (patient) contributes one case per interval in which it remains in the risk set. Thus, for example, patient 1 contributes two cases; one for the first examination period in which no recurrence occurred, and one for the second examination period, in which a recurrence was recorded. Patient 10, on the other hand, contributes a single case because a recurrence was recorded in the first period. Patients 16, 28, and 34 dropped out of the study after six months, and thus contribute only a single case to the new dataset.

  1. Recall the Generalized Linear Models dialog box and click the Type of Model tab.
    Figure 1. Generalized Linear Models Type of Model tab
    Generalized Linear Models Type of Model tab
  2. On the Type of Model tab, select Interval censored survival as the type of model. This specifies Binomial as the distribution and Complementary log-log as the link function.
  3. Click the Response tab.
    Figure 2. Generalized Linear Models Response tab
    Generalized Linear Models Response tab
  4. On the Response tab, select Result by period as the dependent variable.
  5. Click Reference Category.
    Figure 3. Generalized Linear Models Reference Category dialog box
    Generalized Linear Models Reference Category dialog box
  6. In the Reference Category dialog, select First (lowest value) as the reference category for the response variable.
  7. Click Continue, and then click the Predictors tab.
    Figure 4. Predictors tab
    Predictors tab
  8. On the Predictors tab, select Patient ID, Duration of disease, Treatment group and Period as factors.
  9. Select Age in years as a covariate.
  10. Click Options.
    Figure 5. Generalized Linear Models Options dialog box
    Generalized Linear Models Options dialog box
  11. In the Options dialog, select Descending as the category order for factors. This indicates that the first category of each factor will be its reference category; the effect of this selection on the model is in the interpretation of parameter estimates.
  12. Click Continue, and then click the Model tab.
    Figure 6. Model tab
    Model tab
  13. On the Model tab, select duration through age (when variables are listed in file order) as model terms.
  14. Move period to the top of the list of model effects.
  15. Deselect Include intercept in model.

    Suppressing the intercept and including period as the first model term causes the procedure to fit a full set of dummy variables to capture the period effects.

  16. Click OK.

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