Linear Statistical Models: Regression

Ordinal Predictor Variables

One type of categorical predictor variable is an ordinal variable. If an ordinal predictor has only three or four levels then clearly it should coded using dummy or effect coding. There are times when an ordinal predictor can be treated as if it were interval (this is called quasi-interval) especially if the variable has more than five or six levels.

The trick here is to test whether the categorical coded variable contains information that is not captured by the quasi-interval version. We do this by including both the quasi-interval version and the coded in the same model.

For our example, we will use the hsbdemo dataset and create an ordinal version of the write variable.

Example With Equal Intervals

Note: The dummy coded variable for oread does not contain significant information that is not contained in the ordinal variable.

Linear Statistical Models Course

Phil Ender, 17sep10