contrast_ex1.R

rm(list = ls())
library(contrast)
taps <- c(242, 245, 244, 248, 247, 248, 
242, 244, 246, 242, 248, 246, 245, 247, 
248, 250, 247, 246, 243, 244, 246, 248, 
250, 252, 248, 250, 246, 248, 245, 250)
group <- c(0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 1, 
1, 1, 1, 1, 1, 1, 1, 1, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2)
df1 <- data.frame(taps, group)

m <- lm(taps ~ group , data = df1)
summary(m)

# Is the average finger tapping ability different between the combined 100mg and 200mg caffeine doses
# and the placebo (group 0), i.e.
# Does 0.5(mu1 + mu2) - mu0 = 0 ???
contrast(m, a = list(group=c("1","2")), b=list(group="0"), type = "average")

# is there a difference between group 1 and 2 i.e.
# does mu3 - mu2 = 0 ???
contrast(m, a = list(group=c("1")), b = list(group=c("2")))