In this section, SAS presents Type I and Type III sums of squares and F statistics for their significance
under a particular set of assumptions; namely, that fertilizer and variety should be modeled with fixed
effects, and that the random error terms satisfy their requirements. The F test statistics shown here are not
always the proper results to interpret! This depends on the design of the experiment.
The Type I sums of squares are also called sequential sums of squares. Here, they test:
1. Whether fertilizer is a significant predictor
2. Whether variety is significant when considered in addition to fertilizer
3. Whether the interaction is significant when considered in addition to both fertilizer and variety.