Some researchers and advocates say that further investigation is still needed on whether grocery stores and chain supermarkets in poor neighborhoods are selling produce that is too costly and of poor quality. “Not all grocery stores are equal,” said John Weidman, deputy executive director of the Food Trust, an advocacy group in Philadelphia.

It was difficult to design a study that could rigorously answer the questions: Do poor urban neighborhoods lack places to buy fresh produce and is that contributing to obesity? But Helen Lee of the Public Policy Institute of California, a nonprofit, nonpartisan research organization, found a way. For data on where children lived and went to school and how much they weighed, she used a federal study of 8,000 children. For data on the location of food establishments, she used a data set that compiled all the businesses in the nation and included their sizes and locations.

“I knew where the children lived, so let’s take the middle of that neighborhood,” Dr. Lee said. “What is the nearest grocery store? What is the nearest convenience store?”

She used census tracts to define neighborhoods because they tend to have economically homogeneous populations. Poor neighborhoods, Dr. Lee found, had nearly twice as many fast food restaurants and convenience stores as wealthier ones, and they had more than three times as many corner stores per square mile. But they also had nearly twice as many supermarkets and large-scale grocers per square mile. Her study, financed by the institute, was published in the March issue of Social Science and Medicine.

Dr. Sturm’s study, published in February in The American Journal of Preventive Medicine, had a different design. With financing from the National Institutes of Health, he used data on the self-reported heights, weights, and diets of more than 13,000 California children and teenagers in the California Health Interview Survey. The survey included the students’ addresses and the addresses of their schools. He used a different data set to see what food outlets were nearby. Dr. Sturm found no relationship between what type of food students said they ate, what they weighed, and the type of food within a mile and a half of their homes.

He has also completed a national study of middle school students, with the same result — no consistent relationship between what the students ate and the type of food nearby. Living close to supermarkets or grocers did not make students thin and living close to fast food outlets did not make them fat. The study will be published soon in Public Health.