A randomized, double-blind, placebo-controlled experiment was conducted in which patients with Alzheimer's disease were given either extract of Ginkgo biloba (EGb) or a placebo for one year. The change in each patient's Alzheimer's Disease Assessment Scale - Cognitive subscale (ADAS-Cog) score was measured. The results are given in the table.

(a) Use a chi-square test to compare the prevalence rates at α = 0.05. (The value of the chi-square statistic is χ2s = 10.26.)
(b) Verify the value of χ2s given in part (a).

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Change in ADAS-Cog score -4-I + +4 or to to to or Better -2 +3 Worse EGb 72 16 18 12 11 19 11 Placebo 10 24