To assess the effectiveness of the flu vaccine for county residents,Mr.McCallum wants to administer vaccine injections to all county residents rather than give half of them a placebo injection.Mr.McCallum is most clearly underestimating the importance of

A) testing a large sample.
B) operationally defining his procedures.
C) replicating observations of other researchers.
D) creating a control group.

Mr. McCallum is underestimating the importance of creating a control group. Without a control group, it would be impossible to determine if the effectiveness of the flu vaccine is due to the vaccine itself or to other factors (such as a lower incidence of the flu that year). By giving half of the residents a placebo injection, the effectiveness of the flu vaccine can be compared to the effectiveness of no vaccine at all.