This document contrasts the concepts of interaction and effect modification using formal definitions and examples.
1) Interaction is defined in terms of the effects of two interventions, whereas effect modification is defined as the effect of one intervention varying across strata of a second variable.
2) Effect modification can be present without interaction, such as when a variable is a proxy for another cause. Interaction can also be present without effect modification, such as when two variables interact but one does not truly modify the effect of the other.
3) There are some cases where it is possible to assess effect modification but not interaction, or to assess interaction but not effect modification, depending on how the variables are related causally.