Setting reference levels in partial least squares
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When you include a categorical predictor in your model, Minitab assigns one categorical level as the reference level. Minitab designates the reference level based on the data type.

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.

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.

You can change the default reference level in the Options subdialog box when you use 1, 0 coding. To change the reference level for a categorical predictor, specify the variable followed by the new reference level in the Reference level box. You can specify reference levels for more than one categorical predictor at the same time. If the levels are text or date/time, enclose them in quotes.