Table 1.

Conceptual difference between within- and between-subgroup tests

Subgroup	Group, outcome percentage		Subgroup effect (relative risk reduction)
	Treatment	Control
Category 1	34	40	0.15	↕
Category 2	22 ↔	42	0.48
Category 3	25	35	0.29

The horizontal arrow indicates a within-subgroup test. The results of this test are called a subgroup effect. In our example, the test is performed for every subgroup using a χ² test. The vertical arrow indicates a between-subgroup interaction test. The results of this test are called an interaction. It is not performed in our example.