A researcher finds that college students are more anxious near final exams in December than at the beginning of the semester in September. However, it is not clear whether the anxiety is caused by...

A researcher finds that college students are more anxious near final exams in December than at the beginning of the semester in September. However, it is not clear whether the anxiety is caused by exams or by the change in season. Does this study have a problem with internal validity or external validity?