Ordinal variables — categorical variables with a defined order to the categories,
but without equal spacing between them — are frequently used in social science
applications. Although a good deal of research exists on the proper modeling
of ordinal response variables, there is not a clear directive as to how to model
ordinal treatment variables. The usual approaches found in the literature for
using ordinal treatment variables are either to use fully unconstrained, though
additive, ordinal group indicators or to use a numeric predictor constrained to be