Online doctor ratings and reviews are becoming popular with consumers—about one-third of consumers now use a ratings and review web site to check out a doctor, say Harvard Medical School researchers.

But a lot of popular consumer and healthcare ratings sites such as Yelp.com, Vitals.com, UCompareHealthcare.com, DrScore.com and others are lacking in both the  quantity and quality of physician reviews they offer consumers, says a new study from the Journal of the American Medical Association.

The study, led by Tufts University School of Medicine Dr. Tara Lagu, looked at 28 popular ratings sites that contain physician reviews posted by consumers. In September researchers searched Google for websites that allowed patients to review physicians in the U.S. and used search terms such as “rate my doctor.” All the sites were in English, available to the public, allowed patients to leave reviews, did not require a subscription and allowed searching by physician name.

Next researchers used available lists of registered and active physicians to identify a random sample of 600 physicians from Boston, Portland, Ore., and Dallas and searched each website for reviews for each doctor.

While the survey found 8,133 total reviews, what Lagu and other researchers say is more important is that the median number of consumer ratings per doctor was 7 reviews—mainly a star rating of the doctor on a 1-5 scale. But the survey also found that 34% of the physicians in the research sample had no posted reviews on any site, and that there were written comments about only 21.8%, or 1,784, of them.

The survey concludes that even though more consumers want to use online reviews and ratings to find new doctors or evaluate their quality of care, there aren’t enough online reviews to give patients a clear and balanced picture of most doctors. “These results demonstrate that it is difficult for a prospective patient to find—for any given physician on any commercial physician-rating website—a quantity of reviews that would accurately relay the experience of care with that physician,” Lagu and others write in their survey.

While more big health systems such as Cleveland Clinic, Mayo Clinic and others are beginning to publish their own doctor ratings and reviews—and give patients the chance to rate and comment— Lagu writes that it will be a long time before there is a critical mass of reviews collected online to be of long-term use to patients. “The increase in number of reviews that we observed was not meaningful—most physicians in 2016 still had no more than one review on any site,” the study says.