Combining factors acts like interacting two variables. In other words, like interacting two binary variables to create all four possible combinations.
# Create a binary variable for treatment or control treatment <- gl(2, 1, labels = c("treatment", "control")) # Create a binary variable for female or male gender <- gl(2, 1, labels = c("female", "male"))
# Interact the factors by combining them interaction(treatment, gender)
 treatment.female control.male Levels: treatment.female control.female treatment.male control.male