Table 3.6 shows results of a three-center clinical trial designed to compare a drug to placebo for treating severe migraine headaches. At each center, subjects were randomly assigned to treatments.

a. Describe the associations in the partial tables. Are results similar among centers?

b. Find the P-value for testing that response is conditionally independent of whether a subject receives drug or placebo. Interpret.

c. Calculate and interpret an estimate of the average conditional association between group and response.

d. Test whether the true odds ratio is the same in each center. Interpret.