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Release: June 22, 2000

Observational studies can be as valid as randomized trials

IOWA CITY, Iowa -- Physicians have never considered observational studies to be as valid as randomized controlled trials, leading many of them to disregard observational findings. However, observational studies published in reputable medical journals often give similar results to randomized controlled trials, according to an analysis by University of Iowa researchers.

The investigation has implications for how medical treatments are evaluated for effectiveness. The findings were published in today's issue of the New England Journal of Medicine.

In randomized controlled trials, clinicians randomly assign patients to receive one of two or more medical treatments and subsequently analyze the treatment results. In observational studies, patients are treated as usual by their physician, who decides the treatment course. Researchers become involved post-treatment in analyzing the treatment data.

"We found that for several treatment areas the two types of studies gave similar results," said Arthur J. Hartz, M.D., Ph.D., UI professor of family medicine and co-author of the study. "Our findings suggest that the results from observational studies should not necessarily be dismissed. We should take a closer look at how to integrate observational studies with other types of research."

He added, "There is a tremendous amount of computerized data that has been ignored because of the stigma attached to observational studies. These data sets may provide an untapped resource for the evaluation of medical treatments."

Hartz, who co-authored the study with Kjell Benson, a UI medical student who recently completed his second year, said they had accepted the conventional wisdom that these observational studies give different results than controlled trials and that they set out to investigate what types of observational studies were most likely to be valid. However, as they read research articles, they found that the observational results were surprisingly similar to the results from randomized controlled trials, and they began to pursue that similarity.

"Everyone assumed the question was settled," Benson said. "While randomized controlled trials will always be the gold standard, certain observational studies have utility, being cheaper and easier to conduct, and may be equally reliable."

Using medical databases, including Medline and Cochrane, the researchers found and analyzed observational studies and randomized controlled trials published between 1966 and 1998 that evaluated the same treatments. The researchers analyzed studies of physician-implemented treatments that assessed the difference between two treatments or between one treatment and no treatment. They eliminated studies in which patients had a say in treatment received, such as studies about vitamin use or routine screening for disease.

The investigation resulted in a collection of 136 articles that covered 19 different treatment areas such as hormone replacement therapy for osteoporosis, calcium-channel blocker therapy for coronary artery disease and fertility treatments.

In only two of the 19 treatment areas was the difference between the observational studies and the randomized controlled trials statistically significant. In a few additional treatment areas, there were qualitative differences between the observational studies and the randomized controlled trials, but the patient sample sizes were not adequate to prove a difference. However, in the majority of treatment comparisons, the results from the two types of studies were similar.

"We did not cover the universe of medical studies," Hartz said. "There are many medical treatment areas for which no one has tried to do an observational study. In addition, the majority of the observational studies we assessed were in the higher-quality journals."

The advantage of randomized controlled trials, Hartz said, is that it increases the likelihood that patients who have one treatment are on average similar to patients who have the other treatment. However, the method can be expensive and take considerable time to obtain results.

In observational studies, patients treated with one method are compared to patients treated with another method for the same condition. The study is not considered experimental because the person conducting the research does so only after treatment has been provided by a physician.

"The advantage of observational studies is that the data may already be available or relatively easy to collect," Hartz said. "The disadvantage is that differences in the patient outcomes for the two treatments may be due to differences in the patients. For example, a surgical intervention may be primarily reserved for the sickest patients."

Hartz said that observational trials are not appropriate for the definitive evaluation of treatments that have only modest benefits. Benson noted that, where appropriate, an observational study could be used, then followed by a randomized control trial.

Benson's involvement in the study was made possible through the UI College of Medicine Medical Student Research Program and the Family Medicine Research Program, which are

supported by grants from the federal Health Services and Resources Administration and the National Heart, Lung and Blood Institute.

University of Iowa Health Care describes the partnership between the UI College of Medicine and the UI Hospitals and Clinics and the patient care, medical education and research programs and services they provide.