A psychologist evaluates 60 people at the start of therapy and again at the end of 8 weeks of therapy. She reports that 55 of the 60 are "improved," and concludes that the therapy was effective. One flaw in this study is that it lacks

A) descriptive statistics.
B) a control group.
C) a dependent variable.
D) a hypothesis.

Without a control group, it is impossible to know whether the improvements observed were actually due to the therapy or to other factors such as placebo effects or natural recovery. A control group would allow for a comparison of the outcomes between those who received the therapy and those who did not, which is necessary to determine the effectiveness of the therapy. The other options (A, C, and D) may also be important to consider in a research study, but the lack of a control group is the most critical flaw in this case.