Matthew Is Conducting Research to Learn Whether Chewing Peppermint Gum

Multiple Choice

Matthew is conducting research to learn whether chewing peppermint gum during learning will improve recall if the participants are also chewing peppermint gum when they take a test. Matthew assigns the first 10 people who arrive to his experimental group. He assigns the last 10 people to his control group. What mistake has Matthew already made?

A) He didn't ask if anyone prefers cinnamon gum.
B) He forgot to ask how old each of the subjects is.
C) He failed to use random assignment.
D) He forgot that food (including gum) is always a confound.