Other ANOVA Models

The anovan function also has arguments that enable you to specify two other types of model terms:

• 'nested' argument specifies a matrix that indicates which factors are nested within other factors. A nested factor is one that takes different values within each level its nested factor.

  Suppose an automobile company has three factories, and each factory makes two car models. The gas mileage in the cars can vary from factory to factory and from model to model. These two factors, factory and model, explain the differences in mileage, that is, the response. One measure of interest is the difference in mileage due to the production methods between factories. Another measure of interest is the difference in the mileage of the models (irrespective of the factory) due to different design specifications. Suppose also that, each factory produces distinct car models for a total of six car models. Then, the car model is nested in factory.

  Factory	Car Model
  1	1
  1	2
  2	3
  2	4
  3	5
  3	6

  It is also common with nested models to number the nested factor the same way in each nested factor.

• 'continuous' argument specifies that some factors are to be treated as continuous variables. The remaining factors are categorical variables. Although the anovan function can fit models with multiple continuous and categorical predictors, the simplest model that combines one predictor of each type is known as an analysis of covariance model. Analysis of Covariance describes a specialized tool for fitting this model.