That kind of fog is what Dr. Briggs and the National Center for Complementary and Alternative Medicine, with a budget of $122 million this year, are trying to eliminate. Their trials tend to be longer and larger. And if a treatment shows promise, the center extends the trials to many centers, further lowering the odds of false positives and investigator bias.

For instance, the center is conducting a large study to see if extracts from the ginkgo biloba tree can slow the progression of Alzheimer’s disease. The clinical trials involve centers in California, Maryland, North Carolina and Pennsylvania and recruited more than 3,000 patients, all of them over 75. The study is to end next year.

Image Dr. Josephine P. Briggs and the National Center for Complementary and Alternative Medicine have been conducting clinical trials. Credit... Ken Cedeno for The New York Times

Another large study enrolled 570 participants to see if acupuncture provided pain relief and improved function for people with osteoarthritis of the knee. In 2004, it reported positive results. Dr. Brian M. Berman, the study’s director and a professor of medicine at the University of Maryland, said the inquiry “establishes that acupuncture is an effective complement to conventional arthritis treatment.”

In an interview, Dr. Briggs said another good way to improve clinical trials was to ensure product uniformity, especially on herbal treatments. “We feel we have really influenced the standards,” she said.

Over the years, laboratories have found that up to 75 percent of the samples of ginkgo biloba failed to show the claimed levels of the active ingredient. Scientists doing a clinical trial have a large incentive to fix that kind of inconsistency.

Dr. Briggs said such investments would be likely to pay off in the future by documenting real benefits from at least some of the unorthodox treatments. “I believe that as the sensitivities of our measures improve, we’ll do a better job at detecting these modest but important effects” for disease prevention and healing, she said.