Chapter 19 Stratified 2-by-2 Tables. In Chapter 19:. 19.1 Preventing Confounding 19.2 Simpson’s Paradox (Severe Confounding) 19.3 Mantel-Haenszel Methods 19.4 Interaction. Confounding ≡ a distortion brought about by extraneous variables Word origin: “to mix together”. §19.1 Confounding.
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Word origin: “to mix together”§19.1 Confounding
Restriction – impose uniformity in the study base; homogeneity with respect to potential confoundersMitigating Confounding
. St. Thomas Aquinas Confounding Averroлs
3. Matching – balances confounders
4. Regression models – mathematically adjusts for confounders
5. Stratification – subdivides data into homogenous groups (THIS CHAPTER)
An extreme form of confounding in which in which the confounding variablereverses the direction the association
Crude comparison ≡ head-to-head comparison without adjustment for extraneous factors.
Can we conclude that helicopter evacuation is 35% riskier?
Quite different from crude OR
(direction of association reversed)
Again, quite different from crude RR.
Seriousness of accident (C) is independent risk factor for death (D)
Seriousness of accident (C) is not in the causal pathway (i.e., helicopter evaluation does not cause the accident to become more serious)Accident EvacuationProperties of Confounding
Combine strata-specific RR^s to derive a single summary measure of effect “adjusted” for the confounding factor
Calculate by computer
RR-hatM-H = 0.80 (95% CI for RR: 0.63 – 1.02)
Step A: H0: no association (e.g., RRM-H = 1)
Step B: WinPEPI > Compare2 > A. > Stratified
Step D: P = .063 or P = .2078 (cont-corrected) evidence against H0 is marginally significant
Mantel-Haenszel methods are available for odds ratio, rate ratios, and risk difference
Same principle apply (stratify & use M-H to summarize and tests
Covered in text, but not covered in this presentation
Too heterogeneous to summarize with a single OR
OR-hat2 = 2
OR-hat1 = 60
D. Conclude: Good evidence of interaction avoid MH and other summary adjustments