## construct artificial data-set #### A <- factor(rep(c("low", "med", "high"), each = 2)) B <- factor(rep(c("yes", "no"), 3)) set.seed(12) art <- data.frame(A = A, B = B, y = 10 + ifelse(B == "yes", 6, 0) + rnorm(6)) art table(art$A, art$B) ## overview of number of observations fit <- aov(y ~ A + B, data = art) summary(fit) ## what happens if we try to fit the *full* model? fit.full <- aov(y ~ A * B, data = art) summary(fit.full) ## no error term left, hence no tests reported