I sas this example in the book R in action, the codes in R are as follows:
library(multcomp)
attach(cholesterol)
table(trt)
trt
aggregate(response, by=list(trt), FUN=mean)
aggregate(response, by=list(trt), FUN=sd)
fit<-aov(response ~ trt)
summary(fit)
tuk<-glht(fit, linfct=mcp(trt="Tukey"))
plot(cld(tuk, level=.05), col="lightgrey")
It is written in the book that:
From these results, you can see that taking the cholesterol-lowering drug in 5 mg doses four times a day was better than taking a 20 mg dose once per day. The competitor drugD wasn’t superior to this four-times-per-day regimen. But competitor drugE was superior to both drugD and all three dosage strategies for our focus drug
What I understund is:
four times a day is better than one time a day, because the mean is higher. (mean is the right value to watch, isn't?)
DurgE is the best among all, because of the mean.
What I could not understand is:
Why is four times a day better than drugD? The mean of drugD is higher.
How could four times a day be compared with drugD? They are not signifikantlly different. (they both have the alphabet c in this graphic)