Chapter 18 Part C: Matched Pairs 7/30/2019 Chapter 18 Part C: Matched Pairs 7/30/2019 Basic Biostat

§18.6 Matched Pairs Binary data Match on extraneous factors that can confound results 1:1 match 7/30/2019

Case-Control Example Case-control study on colon polyp re-occurrence Cases & controls matched on age, clinic, sex and other potentially confounding factors Let “exposure” = low fruit/veg consumption Ascertain exposure in case-control pairs 45 pairs: case exposed and control not exposed 24 pairs: control was exposed and case not exposed Case exposed Case non-exposed Control exposed ? 24 Control non-exposed 45 7/30/2019

Matched-Pairs Odds Ratio Case E+ Case E− Control E+ a b Control E− c d Interpret like any other odds ratio i.e, as a relative risk 7/30/2019

Colon polyp recurrence / low fruit illustrative data Case E+ Case E− Cntl E+ ? 24 Cntl E− 45 88% higher risk w/ low fruit/veg consumption 7/30/2019

Confidence Interval for OR Matched Pairs Case E+ Case E− Control E+ a b Control E− c d 7/30/2019

95% CI for OR Colon polyps illustrative example Interpret like any other CI 7/30/2019

WinPEPI > PairEtc > A. Input WinPEPI produces these superior confidence limits 7/30/2019

Cohort Matched-Pairs Exposed and non-exposed matched pairs Smoking and non-smoking twins (exposure = smoking) Disease = earlier death 17 twin-pairs in which smoking twin died first 5 twin-pairs in which non-smoking twin died first Smoker D+ Smoker D− Non-smoker D+ 5 Non-smoker D− 17 7/30/2019

Hypothesis Test A. H0: OR = 1 (“no association”) B. McNemar’s z (or equivalent chi-square) C. P-value 7/30/2019

Twins Mortality Example Conclude: Good evidence against H0 (or other reasonable P-value interpretation) 7/30/2019