Generalizing from the results of randomized studies of treatment: Can non-randomized studies be of help?

Different patients who have the same illness may receive different forms of treatment. By documenting their treatment and the progression and complications of their illness, an attempt can be made to draw inferences regarding the relative impact of these different treatments. However, the interpretation of such comparisons can be compromised by underlying differences in the likelihood of the various outcome events among the patient treatment groups. Differences of opinion exist with respect to the frequency and magnitude of the confounding that may arise from these underlying differences.