Cohort studies: prospective versus retrospective.

Cohort studies form a suitable study design to assess associations between multiple exposures on the one hand and multiple outcomes on the other hand. They are especially appropriate to study rare exposures or exposures for which randomization is not possible for practical or ethical reasons. Prospective and retrospective cohort studies have higher accuracy and higher efficiency as their respective main advantages. In addition to possible confounding by indication, cohort studies may suffer from selection bias. Confounding and bias should be prevented whenever possible, but still can exert unknown effects in unknown directions. If one is aware of this, cohort studies can form a potent study design in nephrology producing, in general, highly generalizable results.