A clinical trial is run to evaluate the effectiveness of a new drug to prevent p

Question

A clinical trial is run to evaluate the effectiveness of a new drug to prevent preterm delivery. A total of n=250 pregnant women agree to participate and are randomly assigned to receive either the new drug or a placebo and followed through the course of pregnancy.   Among 125 women receiving the new drug, 24 deliver preterm and among 125 women receiving the placebo, 38 deliver preterm. Construct a 95% confidence interval for the difference in proportions of women who deliver preterm.

A. (-0.35, 0.07) or (-0.07, 0.35)

B. (-0.1, 0.09) or (-0.09, 0.1)

C. (-0.22, 0.00) or (0.00, 0.22)

D. (-0.43, 0.2) or (-0.2, 0.43)

Explanation / Answer

Standard error, SE = sqrt{ [p1 * (1 - p1) / n1] + [p2 * (1 - p2) / n2] } = 0.054164

Critical z value = =/-1.96

95% CI = (p1-p2) +/- zcrit*SE = 0.112 +/- 1.96*0.054164 = (0.00, 0.22)

Option C

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