Identifying Effect modification: Stratification Lung cancer Yes Lung cancer No Alcohol Yes 180 (A) 120 (B) No alcohol No 120 (C) 180 (D) Lung Ca Smokers Lung Ca Non-smokers Yes No Yes No Alcohol Yes 140 20 40 100 No alcohol No 70 50 50 130 Crude OR=AD/BC =180x180/120x120 = 2.25 OR among smokers =140x50/70x20 =5 OR among non-smokers =40x130/100x50 =1 Crude OR Stratified OR
Interpretation Crude analysis – alcohol was associated with an risk of lung cancer Stratified analyses Alcohol consumption was associated with an risk of lung cancer among smokers , but not among non-smokers Smoking modifies the effect of alcohol consumption on lung cancer risk Stratum specific results must be presented if effect modification is present
Confounding vs. Effect Modification Confounding • Nuisance that you must get rid of (biases the occurrence relation) • Similar effect in different strata Effect modification • Important aspect of the occurrence relation and must be understood in order to draw specific conclusions • Different effects in different strata
Confounding vs. Effect modification Compare crude effect estimates with stratum- specific effect estimates All Men Women Confounding EM cRR=4.5 RR=1.0 RR=1.0 + - cRR=5.5 RR=9.0 RR=1.5 (-) + cRR=1.0 RR=4.0 RR=4.0 + - cRR=4.0 RR=4.0 RR=4.0 - -
Framework for identifying confounding and Effect Modification Yes Yes No N o Yes Is there an association between an exposure and outcome? Is it due to confounding or bias Is the association of similar magnitude in subgroups of the population Spurious association Effect modification present No effect modification Adapted from Szklo M and Nieto J. Epidemiology: Beyond the basics. Second edition
Framework – Confounding vs. EM II R e p o r t c r u d e O R / R R N o c o n f o u n d i n g S t r a t a O R / R R s i m i l a r t o c r u d e A d j u s t , a n d p r e s e n t a d j u s t e d O R / R R C o n f o u n d i n g p r e s e n t S t r a t a O R / R R d i ff e r e n t f r o m c r u d e I d e n t i c a l O R / R R a c r o s s s t r a t a R e p o r t O R / R R f o r e a c h s t r a t u m E ff e c t m o d i fi c a t i o n D i ff e r e n t O R / R R a c r o s s s t r a t a E x a m i n e O R s / R R s i n e a c h s t r a t u m E x a m i n e c r u d e O R / R R
“The world is richer in associations than meanings, and it is the part of wisdom to differentiate the two” – John Barth
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- Fall '15