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Joni’s Section

This article discusses the impact of causal parameters and non-collapsibility of rate ratios in exposed populations. It explores how genotype and chemical exposure affect rates and person-time distribution.

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Joni’s Section

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  1. Joni’s Section 01/26/10

  2. When focusing on causal parameters in an exposedsource population (e.g., RR1 = R1/R1 = a/a0), there is no confounding if the total proportion of Type 1 and Type 3 individuals is the same in exposed and unexposed groups. • In this situation, the risk of disease in the unexposed group (R0) is equal to what the risk would have been in the exposed group in the absence of exposure (R1).

  3. Table 4-1 p 60 ME2 (Rothman and Greenland). An elementary model of causal types and their distribution in two distinct cohorts Causal risk difference: (p1+p2) - (q1+q3) get disease in cohort 1(=exposed)get disease in cohort 0 (=unexposed) Causal risk ratio: (p1+p2) (q1+q3) Causal odds ratio: (p1+p2) / (p3+p4) (q1+q3) / (q2+q4) NOTE: if q1 + q3p1 + p3 then q1+q3 cannot be exchanged or substituted for p1+p3 the association measure (risk comparisons) are confounded by the discrepancy between these two quantities 1=gets disease, 0=does not get disease

  4. Cohort of 2000 men • Genotype the causes headaches at a rate of: • 0.2 cases per Person-Yr (PY) in those w/out G • 2 cases per PY in those with G • Imagine scenario where entire cohort experiences same chemical exposure that increase rates • 0.4 cases per Person-Yr (PY) in those w/out G • 4 cases per PY in those with G

  5. Why Non-Collapsibility? • Chemical Exposure and Genotype affect both the numerator and denominator of the rates • Chemical Exposure affected how accrued Person Time • Person time is an outcome. When the exposure and covariate (Here Genotype) affect the outcome under study or competing risk, they will alter person-time distribution

  6. Key Points • Non-collapsibility of rate ratio : • Can happen with or without confounding • Can occur when Outcome is rare • Ex. When competing risks are present • Person time is not affected when everyone remains at risk throughout follow-up

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