As part of a study of risk factors for stroke, 155 women who had experienced a hemorrhagic stroke (eases) were interviewed. For each case, a control was chosen who had not experienced a stroke; the control was matched to the case by neighborhood of residence, age, and race. Each woman was asked whether she used oral contraceptives. The data for the 155 pairs are displayed in the table. "Yes" and "No" refer to use of oral contraceptives.

To test tor association between oral contraceptive use and stroke, consider only the 43 discordant pairs (pairs who answered differently) and test the hypothesis that a discordant pair is equally likely to be "yes/no" or "no/yes." Use McNemar's test to test the hypothesis that having a stroke is independent of use of oral contraceptives against a non directional alternative at α = 0.05.

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Case No Yes Control No 107 30 Yes 13 5