However, unlike experimental studies, observa¬tional studies do not allow the investigator to manipu¬late the specific exposure or experience or, often, to control or limit the effects of other extraneous factors that may influence the development of disease. For example, life stress is known to be related to depression. Those of low socioeconomic status also have high depression rates. Because persons of low socioeconomic status frequently experience greater life stresses, the relationship between stress and depression is difficult to assess. Therefore, the con¬founding factor of socioeconomic status makes it more difficult to demonstrate the effect of stress on depres¬sion