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1、Matthew Fox Advanced Epidemiology,Lecture 8: Selection Bias, Matching, & Control Selection,What is selection bias?,Which studies can have selection bias: cohort or case control?,Selection bias or confounding?,Comparison of mortality among office workers and longshoremen from MI Comparison is biased
2、because those who self-select into longshoremen are fitter which leads to less MI What is the bias?,In a case control study, can we match cases to controls based on exposure?,If we match, do we need to adjust for the matched factor?,What is overmatching?,Misclassification Summary I,#1 Non-differenti
3、al and independent misclassification of dichotomous exposure or disease (usually) creates an expectation that estimates of effect are biased towards the null. #2 Non-differential and independent misclassification of a covariate creates an expectation that the relative risk due to confounding is bias
4、ed towards the null, yielding residual confounding.,Misclassification Summary II,#3 Errors due to misclassification can be corrected algebraically #4 Differential misclassification yields an unpredictable bias of the estimates of effect (still correctable). #5 There are important exceptions to the m
5、antra that “non-differential misclassification biases towards the null.”,This Session,Selection bias Definition & control Matching Cohort vs. Case-control studies When to adjust, when not to adjust Control selection Adjustment Is it possible?,Selection bias definition,Distortions of the estimate of
6、effect arising from procedures to select subjects and from factors that influence participation Common element is that the exposure-disease relation is different among participants than among those theoretically eligible Observed estimate of effect reflects a mixture of forces affecting participatio
7、n and forces affecting disease occurrence,Separate from Confounding,Cohort studies dont have selection bias at entry even if subjects self select Selection into cohort can create confounding, but this can be undone by adjustment Or becomes an issue of generalizablity Cohort studies/RCTs can have sel
8、ection bias at end through differential LTFU Some can be undone if we know enough about the selection mechanism,Selection bias Fallacy,Formerly frequently viewed as disease-dependent selection forces Exposure-dependent selection forces were thought to be confounders or part of the population definit
9、ion. Sometimes selection factors can be controlled as if they were confounders For example, matched factors in case-control studies and two-stage studies. However, not all selection factors related to exposure can be so treated,Selection bias Adjust for selection proportions,Selection bias Simple me
10、thod,Selection bias,Selection bias,OR = 50/4000 / 40/8000 = 2.5,Selection bias,Selection bias,https:/ of Selection Bias,Selection forces dont create bias if they are not related to both exposure and disease,Selection bias Simple method,Selection bias,Selection bias,OR = 50/4000 / 100/8000 = 1,Select
11、ion bias,Selection bias,Selection bias,OR = 50/4000 / 50/4000 = 1,Selection bias,Selection bias,Selection bias,Selection bias,OR = 50/10000 / 40/8000 = 1,Selection bias,Sounds like confounding, but this time E and D affect Selection,Selection Bias Occurs When Selection is Related to Both the Exposur
12、e and the Outcome,Remember back to common causes and common effects (Hernn 2004),Selection Bias in a Case Control Study:,Case controls study of the relationship between estrogens and myocardial infarction Cases are those hospitalized for MI Controls are those hospitalized for hip fracture Could this
13、 cause selection bias?,Selection Bias in a Case Control Study:,E= estrogens D = myocardial infarction F= hip fracture C = selection into study,Selection bias occurs because we condition on a common effect of both E and D,Selection Bias in a Cohort Study:,Cohort study of relationship between HAART an
14、d progression to AIDS LTFU occurs more among those with low CD4 LTFU occurs more among those with AIDS But now selection out occurs before AIDS Could this cause selection bias?,Selection Bias in a Cohort Study: Differential LTFU,E = ART, D = AIDS, L = vector of symptoms U = True immunosuppression (u
15、nmeasured) C= Drop out (LTFU),Selection bias occurs because we condition on a common effect of both E and a common cause C and D,Selection Bias in a Cohort Study: Differential LTFU,E = ART, D = AIDS, L = vector of symptoms U = True immunosuppression (unmeasured) C= Drop out,Selection Bias vs. Confou
16、nding,Bias is a systematic difference between the truth and the observed PrYa=1=1 - PrYa=0=1 PrY=1|a=1 - PrY=1|a=0 Separate from random error which is not structural Using DAGs we can see the common structures Confounding = common causes (directly or through other mechanisms) Selection bias = conditioning on common effects,To see the difference,Comparison of mortality among office workers and longshoremen
