You use a collection parameter to define a set of variables that are treated as a single effect that has multiple degrees of freedom. The variables specified in the vars subparameter can be continuous or classification variables. Columns in the design matrix that are contributed by a collection effect are the design columns of its constituent variables in the order in which they appear in the definition of the collection effect. If you specify the details subparameter, then a table that shows the constituents of the collection effect is displayed.