ABSTRACT
In most experiments there are several variables that possibly affect the response and thus should be included in the statistical model. In the previous chapters we have seen several examples of models and data analyses but there has been only one explanatory variable. Linear regression dealt with the description of one quantitative variable as a linear function of another quantitative variable and one-way analysis of variance modeled the average effect of a quantitative variable for a number of pre-determined groups corresponding to a categorical explanatory variable.
