An experiment was conducted to evaluate the success of a methicillin-resistant Staphylococcus aureus (MRSA) vaccine. MRSA can be prevented if subjects regularly wash their hands. The subjects were treated, but only some of the vaccines were successful. The researchers are unsure whether this was a result of hand washing or the vaccine alone. What is this an example of?
A. Confounding
B. The placebo effect
C. Blinding
D. Replication
E. Bias