A crossover experiment with 100 subjects compares two drugs used to treat migraine headaches. The response scale is success (+) or failure (-). Half the study subjects, randomly selected, used drug A the first time they get a migraine headache and drug B the next time. For them, 6 had responses (A+,B+), 25 had responses (A+,B-), 10 had responses (A-, B+), and 9 had responses (A-, B-). The other 50 subjects took the drugs in the reverse order. For them, 10 were (A+,B+), 20 were (A+, B-), 12 were (A-, B+), and 8 were (A-, B-).

a. Ignoring treatment order, compare the success probabilities for the two treatments. Interpret.

b. McNemar's test uses only the pairs of responses that differ. For this study, Table 9.11 shows such data from both treatment orders. Explain why a test of independence for this table tests the hypothesis that success rates are identical for the two treatments. Analyze these data, and interpret.

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Table 9.11 Treatment Treatment That Is Better Order First Second A then B 25 10 B then A 12 20