To test the association between risk factors and disease, which of the following is the weakest study design?

Ecological studies, as they donot take into consideration individual data of exposure & diseases, have poorest evidence of determining association. Hence they are considered as weak studies. These are done at point of time & commonly used in nutritional surveys. Order from strong to weak study determining association:Randomized Control Trial > Retrospective Coho Study > Prospective Coho Study > Case control study > Cross sectional study > Ecological study.