Generalized Linear Models Reference Category

For binary response, you can choose the reference category for the dependent variable. This can affect certain output, such as parameter estimates and saved values, but it should not change the model fit. For example, if your binary response takes values 0 and 1:

• By default, the procedure makes the last (highest-valued) category, or 1, the reference category. In this situation, model-saved probabilities estimate the chance that a given case takes the value 0, and parameter estimates should be interpreted as relating to the likelihood of category 0.
• If you specify the first (lowest-valued) category, or 0, as the reference category, then model-saved probabilities estimate the chance that a given case takes the value 1.
• If you specify the custom category and your variable has defined labels, you can set the reference category by choosing a value from the list. This can be convenient when, in the middle of specifying a model, you don't remember exactly how a particular variable was coded.

How To Specify a Reference Category for Generalized Linear Models

1. From the menus choose:

   Analyze > Generalized Linear Models > Generalized Linear Models...

   ...or Generalized Estimating Equations.

2. In the Response tab, select a dependent variable and select Variable represents binary response or number of events.

Select Binary response and click Reference Category.