Stratification A case-control study was conducted to determine the association between birth control pills and heart attack in females. Table presents 2x2 tables stratified participants age. 40-49- years stratum 50-59- years stratum Used birth Heart attack Heart attack Used birth Heart attack Heart attack control pills - yes - no control pills - yes - no Yes 81 171 Yes 11 10 No 13 111 No 37 134