Confounding Effect modification is a biological phenomenon in which the

Confounding effect modification is a biological

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Effect Modification vs. Confounding Effect modification is a biological phenomenon in which the exposure has a different impact in different circumstances
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Confounding is a form of “bias” Confounding Obesity/mastitis example Overall, obesity associated with mastitis (crude OR= 1.89) Obese Normal Mastitis + 50 30 Mastitis - 150 170
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Effect modification (interaction) Exposure-disease association differs at different levels of the effect modifier Example: case-control study E = physical activity ≥ 2,500 kcals/< 2,500 kcals D = myocardial infarction Potential effect modifier = alcohol consumption (yes/no) Is the association between physical activity and MI “modified” by background alcohol consumption?
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Let’s think in line graphs How does “risk” change as the level of exposure to the confounder changes Predicted probability of MI decreases as physical activity increases in persons who do not consume alcohol. Predicted probability of MI increases as physical activity increases in persons who do consume alcohol.
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Contrast – confounding: age is a confounder older cows have a higher probability of mastitis, but the association between obesity and mastitis remains the same at different levels of age. Confounding & EM Parallel lines indicate confounding Confounder is associated with disease Old cows are more likely to have mastitis Line for old cows is higher on the graph Effect of exposure on disease is the same at different levels of the confounder Stratum-specific ORs or RRs are equal Results in parallel lines Assuming the association is linear… Parallel lines indicate confounding Stratum-specific ORs/RRs = 1 Lines are horizontal Stratum-specific ORs/RRs > 1 Lines have an upward slope Stratum-specific ORs/RRs < 1 Lines have a downward slope Non-parallel lines indicate EM E – D association differs at different levels of the effect modifier Physical activity & MI study Negative association for alcohol = no Downward sloping Positive association for alcohol = yes Upward sloping
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Stratified analysis Take a “weighted average” of the stratum specific estimates, and we’ve done away with the confounding! The Mantel-Haenszel method provides a pooled odds ratio across the strata of fourfold tables Mantel-Haenszel Estimator Crude OR/RR falls outside range of stratum-specific ORs/RRs Then we shouldn’t use that “crude” number in our study Mantel-Haenszel Estimator Measure of the average effect of exposure across all strata (accounts for confounding and EM)
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  • Mastitis, Confounding, Case-control study

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