EXAMPLE 7.20

How different are the calcium and placebo groups? We estimate that the effect of calcium supplementation is the difference between the sample means of the calcium and the placebo groups, mm Hg. A 90% confidence interval for uses the critical value t* = 1.729 from the distribution. The interval is

We are 90% confident that the difference in means is in the interval. The calcium treatment reduced blood pressure by about 5.3 mm Hg more than a placebo on the average, but the margin of error for this estimate is 5.6 mm Hg.