The researchers conducted a randomized controlled trial. While the patients had fewer hospital stays, the decline in admissions was the result of a phenomenon known as regression to the mean, said Amy Finkelstein, an economist at M.I.T. who led the study. Patients with extremely high medical costs tend to see their expenses naturally decline over time, becoming closer to the average.

Such regression is “the Achilles’ heel of a lot of health research,” said Amitabh Chandra, a health economist at Harvard University, who said most studies were based on observing patients and seeing how they did before and after an intervention. The patient in the hospital will be cured of pneumonia, regardless of what else happened.

“All the observational studies will find huge effects,” he said.

When Dr. Brenner, now an executive with UnitedHealth Group, started the Camden Coalition, the answer seemed more straightforward. Camden was one of the nation’s poorest cities, and by identifying those patients who were in and out of the hospital, he saw firsthand how chaotic and disjointed their care was. Many were being prescribed too many medications, and they were often simply too overwhelmed to figure out how to navigate the system.

“We needed to break down the silos and coordinate their care,” he said. “We saw improvements.”

The concept took off. “Hot spotting has enjoyed tremendous, widespread appeal,” said Mary D. Naylor, a professor at the University of Pennsylvania School of Nursing who is leading similar efforts to reduce hospitalizations among Medicare patients. Her program has shown promising results in randomized studies.

In retrospect, assigning a team to coordinate the care of the sickest patients, with no quick access to housing or treatment programs for substance abuse, “made it seem like an impossible task,” Dr. Naylor said. Camden’s program targets patients with the most serious social and medical challenges. One individual struggled with addiction for decades and was incarcerated for much of his life. When he was released, he would bounce from hospitals to treatment programs. After developing a wide range of medical problems, he was hospitalized for pneumonia when he was brought into the program.

These patients may need help well before they become super-utilizers, Dr. Naylor said, and the program could be more effective if it were able to find patients who were most likely to benefit from the coalition’s efforts. “I’m hopeful we learn a lot from this,” she said.