Cholesterol Reduction Data Set
Cholesterol reduction for five treatments.
data("cholesterol")This data frame contains the following variables
treatment groups, a factor at levels 1time, 2times,
4times, drugD and drugE.
cholesterol reduction.
A clinical study was conducted to assess the effect of three formulations
of the same drug on reducing cholesterol. The formulations were
20mg at once (1time), 10mg twice a day (2times), and 5mg
four times a day (4times). In addition, two competing drugs
were used as control group (drugD and drugE). The purpose of
the study was to find which of the formulations, if any, is efficacious and how
these formulations compare with the existing drugs.
P. H. Westfall, R. D. Tobias, D. Rom, R. D. Wolfinger, Y. Hochberg (1999). Multiple Comparisons and Multiple Tests Using the SAS System. Cary, NC: SAS Institute Inc., page 153.
### adjusted p-values for all-pairwise comparisons in a one-way layout
### set up ANOVA model
amod <- aov(response ~ trt, data = cholesterol)
### set up multiple comparisons object for all-pair comparisons
cht <- glht(amod, linfct = mcp(trt = "Tukey"))
### cf. Westfall et al. (1999, page 171)
summary(cht, test = univariate())
summary(cht, test = adjusted("Shaffer"))
summary(cht, test = adjusted("Westfall"))
### use only a subset of all pairwise hypotheses
K <- contrMat(table(cholesterol$trt), type="Tukey")
Ksub <- rbind(K[c(1,2,5),],
"D - test" = c(-1, -1, -1, 3, 0),
"E - test" = c(-1, -1, -1, 0, 3))
### reproduce results in Westfall et al. (1999, page 172)
### note: the ordering of our estimates here is different
amod <- aov(response ~ trt - 1, data = cholesterol)
summary(glht(amod, linfct = mcp(trt = Ksub[,5:1])),
test = adjusted("Westfall"))Please choose more modern alternatives, such as Google Chrome or Mozilla Firefox.