In a randomized, double-blind, crossover experiment, researchers randomly assigned 20 volunteer hospital employees to either a low-fiber or low-fiber treatment group. The subjects followed the diets for six weeks. After two weeks on their normal diet, all patients crossed over to the other treatment group for another six weeks. The total serum cholesterol (in mg/dl) was measured on each patient before the first treatment, at the end of the first six week treatment, and at the end of the second six week treatment.
A data frame with 20 observations on the following 4 variables.
total serum cholesterol before treatment
total serum cholesterol after the high fiber diet
total serum cholesterol after the low fiber diet
factor to identify order of treatment, with two
Ramsey, F.L. and Schafer, D.W. (2002). The Statistical Sleuth: A Course in Methods of Data Analysis (2nd ed), Duxbury.
Swain, J.F., Rouse, I.L., Curley, C.B., and Sacks, F.M. (1990). Comparison of the Effects of Oat Bran and Low-fiber Wheat on Serum Lipoprotein Levels and Blood Pressure, New England Journal of Medicine 320: 1746–1747.
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str(case1602) subjects <- 1:20 ordersubjects <- order(case1602$Baseline) plot(1:20, case1602$Baseline[ordersubjects], pch=24, xlab="Subjects (Ordered According to Baseline Cholesterol)", ylab="Total Serum Cholesterol (mg/dl)") points(1:20, case1602$Lofiber[ordersubjects], pch=19, col=5) points(1:20, case1602$Hifiber[ordersubjects], pch=21, col=3) legend(1,245,legend=c("Baseline","After Low Fiber Diet","After High Fiber Diet"), pch=c(24,19,21),col=c(1,5,3)) diff <- with(case1602, Hifiber-Lofiber) plot(subjects, diff, pch=ifelse(case1602$Order=="HL",19,21)) abline(h=0) t.test(diff ~ Order, case1602) # Test for order of treatment effect t.test(diff) # Test for treatment effect
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