Setting Reference Levels and Reference Events
main topics 

Reference levels for categorical predictors

When you include a categorical predictor in your binary logistic regression model, Minitab assigns one categorical level as the reference level. Minitab designates the reference level based on the data type and coding scheme. You can choose the coding scheme in the Coding subdialog box.

For -1, 0, 1 coding:

·    For numeric categorical predictors, the reference level is the level with the largest numeric value.

·    For date/time categorical predictors, the reference level is the level with the latest date/time.

·    For text categorical predictors, the reference level is the level that is last in alphabetical order.

For 1, 0 coding:

·    For numeric categorical predictors, the reference level is the level with the least numeric value.

·    For date/time categorical predictors, the reference level is the level with the earliest date/time.

·    For text categorical predictors, the reference level is the level that is first in alphabetical order. If you defined a value order for a text categorical predictor, this rule does not apply. Minitab designates the first value in the defined order as the reference. See Ordering Text Categories.

If you choose 1, 0 coding, you can change the default reference level in the Coding subdialog box.

Regression creates a set of design variables for each categorical predictor in the model. See Coding categorical predictors in Regression for examples.

 Reference event for the response variable

When your data are in binary format or frequency format, Minitab needs to designate one of the response values as the reference event. The default reference event depends on the data type:

·    For numeric factors, the reference event is the greatest numeric value.

·    For date/time factors, the reference event is the most recent date/time.

·    For text factors, the reference event is the last in alphabetical order.

You can change the default reference event in the main dialog box. To change the event, specify the new event value in Response event. To change the order of response values in ordinal logistic regression, specify the new order in the Order of the response values box.

If you have defined a value order for a text factor, the default rule above does not apply. Minitab designates the last value in the defined order as the reference event. See Ordering Text Categories.