Even though the overall consumer price index did not change in April, indicating the absence of any general inflation or deflation, prices in several categories of merchandise have fallen over the last several months.
Psychologist: In a study, researchers gave 100 volunteers a psychological questionnaire designed to measure their self-esteem. The researchers then asked each volunteer to rate the strength of his or her own social skills. The volunteers with the highest levels of self-esteem consistently rated themselves as having much better social skills than did the volunteers with moderate levels. This suggests that attaining an exceptionally high level of self-esteem greatly improves one's social skills.
The psychologist's argument is most vulnerable to criticism on which of the following grounds?