When strict protocols are followed for making diagnoses, applying treatments, and assessing clinical changes, observational studies can be as helpful as RCTs.

To examine whether conclusions of observational studies differ from those of RCTs, Arthur J. Hartz and Kjell Benson compared observational studies and RCTs on cardiovascular and other diseases published since 1984. Surveying 3,868 observational studies, they found 19 for which there was also an RCT for similar problems. Only one of these pairs had a discrepancy, and this was just in the magnitude of the effects of treatment.

Another review found similar concurrences (Lawler, et al).

In many other parts of the world (outside the US), observational studies are considered sufficient for accepting treatment efficacies.

A particular advantage of observational studies is that they cost far less, are easier to run than RCTs, and provide answers to questions of efficacy much more quickly.

Helpful refs:

Benson, Kjell and Hartz, Arthur J. A comparison of observational studies and randomized, controlled trials, New England J Medicine 2000, 342(25), 1878-1886.

Black, N. Why we need observational studies to evaluate the effectiveness of health care, British Medical Journal 1996, 312, 1215-1218