His daughter Hannah Klein confirmed the death.

As World War II was ending, Professor Klein, widely regarded as a brilliant theorist, disputed the conventional wisdom that the postwar period would drive the American economy back into a lengthy depression. Using his econometric models based on mathematical equations, he correctly forecast a flourishing economy built on surging demand for consumer goods and housing.

Though he often testified before federal bodies and served as an economic adviser to Jimmy Carter during his 1976 presidential campaign, Professor Klein chose to remain in academia — he taught economics at the University of Pennsylvania for 33 years — and rejected an offer to join the Carter administration.

"I am just an academic giving advice," he told People magazine in 1976. "If you are a technician and are asked for help, it is a social obligation of citizenship to give it."

Professor Klein's use of vast survey data to build statistical economic models for the United States and several other countries has been adopted by economists around the world. "Few, if any, research workers in the empirical field of economic science have had so many successors and such a large impact as Lawrence Klein," the Nobel committee wrote in awarding him the Nobel Memorial Prize in Economic Science.

Jere R. Behrman, a professor of economics and sociology at Penn and a longtime colleague, said Professor Klein's work was built on the idea that the economy is a set of complex organisms — millions of people, millions of households, corporations, government and other entities — and that it is important to have simple models of these economies to understand their essence. Such models, he said, allow economists "to make predictions about what is likely to happen in the economy if there is a significant change in international markets, such as an increase in the price of petroleum."

"Before Klein," he added, "there had been very little work on these aggregate models."

Lawrence Robert Klein was born in Omaha on Sept. 14, 1920, the second-born of three children of Leo Byron Klein, an office clerk, and the former Blanche Monheit. He attended public schools there and became fascinated with economics at a young age. "Although I was not aware of it at the time, growing up during the Great Depression was to have a profound impact on my intellectual and professional career," he wrote in an autobiographical essay for the Nobel committee.

At the same time, he developed an obsession for baseball and became a batboy for a minor league team in Omaha. But his dream of a baseball career ended when, at age 10, he was hit by a car and suffered a badly broken right leg, an injury that affected him for the rest of his life, Ms. Klein said. (At Penn, he became a passionate Phillies fan, she said.)

After moving with his family to San Francisco, he received an undergraduate degree in economics from the University of California, Berkeley, then pursued graduate studies at the Massachusetts Institute of Technology, becoming one of its first Ph.D. students in economics and earning his doctorate in just two years.

At M.I.T., Professor Klein was a protégé of the economist Paul A. Samuelson, a professor only five years his senior. Professor Samuelson, the first American to win the Nobel in economic science, in 1970, became his dissertation adviser and provided what Professor Klein called "an unforgettable experience."

He left M.I.T. in 1944 to join the econometrics team at the Cowles Commission of the University of Chicago. Its director, Jacob Marschak, gave him the assignment of reviving the early attempts at econometric model building that had been made by Jan Tinbergen, a Dutch economist and the 1969 Nobel laureate.

In Chicago, Professor Klein was surrounded by economic heavyweights like Herbert A. Simon, Trygve Haavelmo and Theodore Anderson. It was also there that he met Sonia Adelson, an economics student from Newport, R.I., whom he married. It was his second marriage.

His wife survives him, as do three daughters, Hannah Klein, Rebecca Klein Kennedy and Rachel Klein; a son, Jonathan; seven grandchildren; and four great-grandchildren.

Daniel E. Slotnik contributed reporting.