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The difference between ‘Effect Modification’ & ‘Confounding’

Posted on June 4, 2015 by Deevia Kotecha

Tutorials and Fundamentals
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Students can often struggle to understand the difference between effect modification and confounding. In order to help with this problem I have put together a simple guide to explain ‘effect modification’ and ‘confounding’ and the difference between the two terms:

Effect Modification

Effect modification is all about stratification and occurs when an exposure has a different effect among different subgroups. Effect modification is associated with the outcome but not the exposure.

For example, imagine you are testing out a new treatment that has come onto the market, Drug X. If Drug X works in females but does not work in males, this is an example of effect modification.


Confounding occurs when a factor is associated with both the exposure and the outcome but does not lie on the causative pathway.

For example, if you decide to look for an association between coffee and lung cancer, this association may be distorted by smoking if smokers are unevenly distributed between the two groups. It may appear that there is an association between coffee and lung cancer, however if you were to consider smokers and non-smokers separately for each group this would in fact show no association.

What is the difference?

Confounding factors are a “nuisance” and can account for all or part of an apparent association between an exposure and a disease. Confounding factors simply need to be eliminated to prevent distortion of results.

Effect Modification is not a “nuisance”, it in fact provides important information. The magnitude of the effect of an exposure on an outcome will vary according to the presence of a third factor.



Deevia Kotecha

Deevia Kotecha

Fourth year medical student studying at the University of Leicester. I am completing an intercalated degree in Medical Research and my current interests are in Oncology, Cardiovascular Sciences and Academic Medicine. View more posts from Deevia

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