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The difference between ‘Effect Modification’ & ‘Confounding’

Posted on 4th June 2015 by

Tutorials and Fundamentals
pie chart

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

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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No Comments on The difference between ‘Effect Modification’ & ‘Confounding’

  • Thomas J. Dolan

    The distinction you make here is easy to understand, and to remember.

    10th May 2021 at 3:20 am
    Reply to Thomas
  • Dr Chris Antony

    Well it was good
    But could have explained in detail with simple examples

    24th April 2021 at 2:44 pm
    Reply to Dr
  • Tadele A,

    I’m happy with definition and the difference between them

    10th March 2020 at 1:26 pm
    Reply to Tadele
  • Indrajit

    A study was conducted to assess the extrapyramidal side effects of a new antipsychotic drug in patients with schizophrenia. Many of these patients were smokers and some of them were on anticholinergic drugs. What was the role of the anticholinergic drugs in this study?

    6th March 2020 at 2:15 pm
    Reply to Indrajit
  • madalitso

    thanks for a brief but clear explanation

    29th August 2019 at 11:02 am
    Reply to madalitso
  • Young Kwadha

    Thank you for presenting this information with utmost clarity.

    29th August 2019 at 8:16 am
    Reply to Young
  • Manoj B Agravat

    Effect modification is a question related to an exposure: 1) that shows independent of the exposure and outcome 2). Effects the outcome by 10 % more or less by an odds ratio or other risk measure 3) as well as a P value test association with an odds ratio related to independent chi square.

    Reference. Links:
    1. http://www.pharmasug.org/proceedings/2011/SP/PharmaSUG-2011-SP03.pdf
    2. http://support.sas.com/resources/papers/proceedings12/315-2012.pdf

    19th June 2019 at 5:56 pm
    Reply to Manoj
  • Giselle Falconi

    Amazing job! I will be sharing your site!

    30th May 2019 at 6:52 pm
    Reply to Giselle
  • Dr Rakhshinda Tanweer

    Thanks Deevia, your explanation and examples helps me in understanding Effect modification and confounders.Will surely like to keep in touch with you for future .

    14th May 2019 at 7:39 pm
    Reply to Dr
  • Chika Agogbuo

    Thanks for the explanation. You just made my day.

    19th January 2019 at 1:17 am
    Reply to Chika
  • Ahmed Eldib

    Thank you Kotecha for the nice explanation. You said that confounders need to be eliminated to prevent distortion of results. Would you eliminate by not including it in the analysis? or by balancing the smoker and non-smoker participants in both groups?

    14th January 2019 at 6:07 pm
    Reply to Ahmed
  • Sylvia Wassertheil-Smoller

    A clear and well-written explanation!

    30th August 2018 at 7:31 pm
    Reply to Sylvia
  • Bekele Kefeni

    Thank you for brief and clear explanation the action of each and their difference.
    It make me to think in other way to act up on when I will deal with my analysis

    12th August 2018 at 8:35 am
    Reply to Bekele
  • Farid Medleg

    Can you explain what “associated with the outcome and not the exposure” means?
    Also, same goes for “associated with both exposure and outcome but does not lie on the causative pathway.”

    Lots of resources are saying the same thing. There’s no doubt you’re all in agreement; I just don’t understand it and all these explanation don’t seem to clarify. I feel like there should be a “stats for dummies” somewhere out there…

    15th July 2018 at 12:01 am
    Reply to Farid
  • Khalid Masood

    Very nicely explained the diffrrence.Thanks

    9th April 2018 at 7:01 pm
    Reply to Khalid
  • moses mwenda

    this is awesome information I needed.

    9th April 2018 at 10:02 am
    Reply to moses

    Thanks a lot for the clear explanations.

    21st April 2017 at 9:54 am
    Reply to REVERIEN
  • Darren

    I’ve been looking for a good definition of effect modifier for some time and I found it here.

    25th November 2016 at 7:27 am
    Reply to Darren

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