The cause was brain cancer, according to her family.

For generations, ever since it was first published in 1936, Consumer Reports has been a go-to source for shoppers seeking an impartial guide to the hurly-burly of the marketplace.

Some consult its pages only for major expenditures — the replacement of a refrigerator or a lawn mower, for example, or the acquisition of a new car. The annual automobile issue, published in April, is consistently one of the magazine’s most popular editions of the year.

Other readers, the die-hards, consult Consumer Reports for the more everyday purchases of such items as electric toothbrushes and bathroom scales, backpacks and cookie sheets, lightbulbs and batteries. Consumer Reports conducts rigorous product evaluations before bestowing the coveted imprimatur of “CR Best Buy.”

Ms. Karpatkin, a lawyer by training, spent 16 years as outside counsel to Consumer Reports and its publisher, then known as Consumers Union, before she was named president in 1974.

“You just got the feeling they couldn’t be bought, couldn’t be seduced,” she told the New York Times shortly before her retirement in 2001, describing the group as “one of the quintessential do-good organizations.”

Along with activists including Ralph Nader and Joan Claybrook, Ms. Karpatkin became a central figure in the consumer movement of the 1970s and beyond.

During her time as president, Consumer Reports nearly doubled its circulation to 4.2 million. Its website, by the time she left, was one of the largest paid subscription sites on the internet, with approximately 475,000 subscribers, according to the magazine.

Ms. Karpatkin guided the magazine through periods of recession and debt, ultimately growing its operating budget by a factor of 10, to $157 million. She oversaw the construction of an auto-test track and new research laboratories. To gauge the durability of samples, Consumer Reports subjected suitcases to a spinning machine and pounded mattresses with bowling balls severed in half to resemble buttocks.

Nader, who argued that public advocacy took primacy over product testing, left the board of Consumers Union during Ms. Karpatkin’s tenure. She saw both product testing and advocacy as central to the organization’s mission.

During the debate over a health-care overhaul in the 1990s, Ms. Karpatkin, from her position at Consumers Union, pushed for a single-payer health-care system as an alternative to managed competition. Medical services, she noted, are one area in which consumers often do not have the luxury of comparison shopping.

“They cannot shop for doctors,” she told the Columbus Dispatch in 1993. “They cannot shop for hospitals, and they certainly cannot do that when they’re calling 911.”

Having grown up during the Depression, she insisted that Consumer Reports not evolve into an upscale buying guide for the upper-middle class. She was deeply attuned to issues of poverty and, outside her work with the magazine, was a longtime volunteer and board member at the West Side Campaign Against Hunger in New York.

In her vision, Consumer Reports was duty bound to represent the interests of buyers who lacked the money even for the most economical car and who washed their clothes at laundromats rather than in household machines that product testers might evaluate for their spin cycles.

“Consumers would be in trouble if they couldn’t get sound, independent information, and that’s our prime responsibility,” she told the magazine when she stepped down. “We’ve dedicated ourselves to testing and journalism of the highest quality, and we have an impact on the marketplace.”

“But that’s not enough,” she continued. “Where would protections against unsafe products, fraud, misleading advertising, and other things we take for granted come from if consumer advocates hadn’t fought for them? We’re not and never have been solely about which products to buy. We’re also about helping to build the kind of society we want to be consumers in.”

Rhoda Alayne Hendrick was born in Brooklyn on June 7, 1930. Her mother, a homemaker and bookkeeper, and her father, a salesman, were both Jewish immigrants from Eastern Europe.

Ms. Karpatkin was a 1951 graduate of Brooklyn College, where she worked on the school newspaper before discarding her plans for career in journalism. She wanted “to do important things, not report on them,” she told the Times.

After graduating from Yale Law School in 1953, she entered private practice, specializing in civil rights, civil liberties and education cases. Besides Consumers Union, her clients included the American Civil Liberties Union and conscientious objectors to the Vietnam War.

She served two terms as president of Consumers International, a membership organization for consumer activist groups.

Ms. Karpatkin’s husband, Marvin M. Karpatkin, died in 1975 after 23 years of marriage. Her longtime partner Bruno Aron died in 2009.

Survivors include three children, Deborah Karpatkin of Manhattan, Herbert Karpatkin of Brooklyn and Jeremy Karpatkin of Philadelphia; 10 grandchildren; and four great-grandchildren.

Ms. Karpatkin cautioned against the pitfalls of consumerism.

“More and more the advertisements and commercial messages we see equate a happy and satisfied life with owning particular kinds of products or particular products,” she told the Austin American-Statesman in 1999. “And that, to a very large extent, will turn out to be a false message.”

But if one had to shop, it was best to do so in an educated fashion, and in her own outings to stores, Ms. Karpatkin often carried with her the relevant issue of Consumer Reports.

Without knowing who she was, “people will follow me around, asking to borrow it or watching what I buy,” she said. “It’s a wonderful feeling.”