The values of a class variable are called levels .
Class variables can be either numeric or character.
Response variables must be numeric. Main effects used as independent variables test the hypothesis that the mean of the dependent variable is the same for each level of the factor in question, ignoring the other independent variables in the model. Interaction terms in a model test the hypothesis that the effect of a factor does not depend on the levels of the other factors in the interaction. Nested effects test hypotheses similar to interactions, but the levels of the nested variables are not the same for every combination within which they are nested. The crossed list should come first, followed by the nested list in parentheses.