Tjalling C. Koopmans, 74, cowinner of the Nobel Prize in economics in 1975 and a retired professor at Yale University, died Feb. 26 at Yale-New Haven Hospital in New Haven, Conn. The cause of death was not reported.

Dr. Koopmans shared the Nobel award with Leonid V. Kantorovich of the Soviet Union. The two men did separate but parallel work in bringing the tools of mathematics to bear on problems such as increasing production efficiency. They followed different paths in different economic systems, but reached nearly identical conclusions in respect to a technique called linear programming.

In the words of the Swedish Royal Academy, they were chosen for the $145,000 prize "for their contribution to the theory of optimum allocation of resources."

Dr. Koopmans was an originator of econometrics, a branch of economics that tries to measure economic developments. It uses mathematical models to test the validity of various economic theories. Linear programming can help planners reach a given goal -- such as maximizing output -- when confronted by constraints such as a physical limitation on the availability of materials.

In an interview at the time he received the Nobel, Dr. Koopmans said he began working on linear programming while serving on the Allied Combined Shipping Board in Washington in World War II. What he tried to do, he said, was figure out "the best way to make the empty ships travel from where they shed their cargo to the next destination."

As it happened, Kantorovich did his first work on linear programming while trying to solve analagous problems on the Siberian railroad in the 1930s. The two scholars began corresponding with each other about 1960 and first met about five years later.

Born in the Netherlands, Dr. Koopmans graduated from the University of Utrecht in 1933 and earned a doctorate from the University of Leiden in 1936.

He came to this country in 1940 and taught at New York and Princeton universities and the University of Chicago before joining the Yale faculty in 1955. He was the first Alfred Cowles professor of economics when the chair was endowed in 1967.

Dr. Koopmans also served as director of the Cowles Foundation, the research arm of Yale's economics department, from 1961 to 1967. He retired from the university in 1977.

He is survived by his wife, Truus Wanningen Koopmans, and one son, two daughters, and one brother.