Example: Qualitative Interaction - Pregnancy and Coffee Study

Lead Author(s): Jeff Martin, MD

Study of Effects of Caffeine Consumption on Delayed Conception

Stanton provides this example from a study effects of caffeine consumption on delayed conception.

The principal exposure in question is smoking.

Is Caffeine an Intermediary Variable?


Look what happens after we stratify by caffeine use (below).

Do Not Summarize the Effect

Would you think it is appropriate to try to summarize these two effects, 2.4 and 0.7, into one overall number?

Qualitative Interaction

So, we stratified by caffeine use and we saw a qualitative interaction, actually both on the multiplicative (i.e., the risk ratio) and additive (i.e., the risk difference) scales. In the presence of effect modification of this magnitude, effect modification trumps any further consideration of confounding. Stop here. End of story.


Stanton, C. K., & Gray, R. H. (1995). Effects of caffeine consumption on delayed conception. Am J Epidemiol, 142(12), 1322-1329.