For several years, evidence had been mounting that folic acid reduces major birth defects. In a study, doctors enrolled women prior to conception and divided them randomly into two groups. One group, consisting of

2787

women, took daily multivitamins containing 0.8 mg of folic acid; the other group, consisting of

2171

women, received only trace elements. Major birth defects occurred in

31

cases when the women took folic acid and in

48

cases when the women did not. Construct a

95%

confidence interval for the difference between the two population proportions,

p1p2.