For example, a prospective study might feasibly assess decoupling the discussion of organ donation from the announcement of brain death on consent rates. However, all evaluations can include elements that strengthen the methodology of the study and produce more rigorous results. For example, a more valid comparison group often can improve study designs. A more rigorous control would be an external control, for example a hospital not receiving the in-service program. The following general guidelines are helpful for weighing the relative rigor of alternative types of controlled studies.
Randomized studies are stronger than non-randomized studies.
Randomized studies require the assignment of subjects to intervention and control groups based on a chance distribution. This technique is used to diminish subject selection bias in controlled studies. Prospective studies are stronger than retrospective studies. In a prospective study, the investigators conduct an investigation on a group of subjects and analyze the outcomes. In a retrospective study, investigators select groups of subjects who have already been subject to an intervention and analyze how the intervention relates to the outcomes.