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. Large studies are stronger than small studies. The sample size of a study should be large enough to have an acceptable probability of detecting a difference in outcomes, if such a difference truly exists, between the experimental and control groups attributable to the intervention being evaluated. Although larger studies increase the statistical power of the evaluation, there is a point beyond which there are diminishing returns and studies may become unnecessarily costly and inefficient.