Page "Analysis of variance" Paragraph 250

Comparisons, which are most commonly planned, can be either simple or compound.

Simple comparisons compare one group mean with one other group mean.

Compound comparisons typically compare two sets of groups means where one set has two or more groups ( e. g., compare average group means of group A, B and C with group D ).

Comparisons can also look at tests of trend, such as linear and quadratic relationships, when the independent variable involves ordered levels.