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Old 01-05-2012
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Default The difference between Effect modification and Confounding bias

The difference between Effect modification and Confounding bias

People usually mix these two up and the explanations in UW and KQB and other rescourses donít do a good job in clearing the mix up very well. They actually confuse you every time you read them !

I have a simple way to make you understand it immediately !! PROMISE !!


1- Confounding bias :

Alcohol ( exposure ) Oral cancer ( outcome )

Smoking is a confounding bias here. Why ?

Because smoking is related to BOTH the exposure and the outcome. People who smoke will more often than not, also drink, and people who have oral cancer, were probably smokers ( smoking is a known risk factor for oral cancer).



2- Effect modification :

  • OCPs ( exposure ) Breast cancer (outcome )


Family history of breast cancer is an effect modification here.
Can you guess why ?

  • Asbestos ( exposure ) Lung cancer(outcome)
Smoking is an effect modification

  • Estrogens ( exposure ) DVT (outcome )

Smoking is an effect modification


So do you notice the difference ?? The effect modification is ONLY related to the outcome, but NOT to the exposure. Think with me here : smoking does not effect neither asbestos exposure nor estrogen levels or intake,BUT definintly is a risk for lung cancer and DVT !!

Family history of breast cancer has nothing to do with choosing to use OCPs , but definitly has a factor in increasing risk of breast cancer !!

Got it ??!


SEE ??!! Itís so simple but the people explaining statistics sometimes find joy in complicating things and torturing us

Good luck !! If you like this, please pray for me or wish me luck in my step2CK exam.
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The above post was thanked by:
anomali (06-22-2012), docsasmit (08-09-2013), drssbaz (05-31-2012), ehtisham (04-30-2014), Faith-USMLE (03-01-2013), Gikson (01-17-2013), inno (01-05-2012), mitsua (07-12-2012), Oscaryasser (10-21-2013), Pelz (05-31-2012), usmlegusmle (12-16-2012), wertheworld11 (11-18-2012)



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