Predictor

Describes a categorical (factor) or a continuous (covariate) predictor for the model. When describing a factor, it can optionally contain a list of categories and a contrast matrix. Such matrix describes the codings of categorical variables. If a categorical variable has n values, there will be n rows and n-1 or n columns in the matrix. The rows and columns correspond to the categories of the factor in the order listed in the Category element if it is present, otherwise in the order listed in the DataField or DerivedField element. If the Categories element is present and the corresponding DataField or DerivedField element has a list of valid categories, then the list in Categories should be a subset of that in DataField or DerivedField. A contrast matrix with n-1 columns helps to reduce the total number of parameters in the model.