Why Not One-Way ANOVA? The given Minitab display results from measured amounts of DDT in falcons partitioned into nine cells according to one factor of location and another factor of age of the falcon. Each cell includes three DDT measurements. Why can’t we conduct a thorough analysis of the data by simply executing two separate tests using one-way ANOVA (described in Section 11-2), where one test addresses the differences in sites and the other test addresses the differences in age?

That is, why is two-way ANOVA required instead of two separate applications of oneway ANOVA?