#R commands for ancova, data in Table 3.34
rm(list = ls())  # clear the memory
starch = read.table("http://www.stat.ualberta.ca/~wiens/stat568/datasets/starch_table_3_34.dat", header = T, quote= "")


type = as.factor(starch[,1])
strength = starch[,2]
thick = starch[,3]

fit1 = lm(strength ~ thick + type)
summary(fit1) # thickness is very significant; types aren't

# incorrect fit, ignoring the covariate:
fitx = aov(strength ~ type)
summary(fitx) # types now seem significant