Final answer:
The most apparent weakness of Dr. Cavanaugh's study is external validity due to the use of a hardiness measure that was only normed on college students, potentially affecting its generalizability to senior citizens.
Step-by-step explanation:
The most apparent weakness of Dr. Cavanaugh's study examining the relationship between the trait of hardiness and senior citizens' compliance with medication regimes is external validity.
External validity refers to the extent to which the findings of a study can be generalized to other populations, situations, or times. In this case, the measure of hardiness was only normed on college students and may not accurately reflect the hardiness of a different demographic group, in this case, senior citizens. As a result, there is a potential issue with external validity because the tool used to measure hardiness may not be appropriate for use with the senior population under study, which could affect the study's generalizability to this different group.