American industries could raise their productivity 15 to 40 percent and compete better with Japan by concentrating on design and reducing the "hidden plant" that studies and replaces rejected units, a quality control expert told scientists here yesterday.

Japan's rigorous quality assurance programs begin with the design, but U.S. attention typically involves only the finished product, said Armand V. Fiegenbaum, chief executive officer of the General Systems Co. Inc., a consulting firm. He told a session of the American Association for the Advancement of Science that the traditional measures of productivity per work hour have never included the 15 to 40 percent of a plant's effort, the "hidden plant," that has to handle rejected products.

Fiegenbaum and a panel of other experts agreed that nothing fundamental stands in the way of a resurgence in U.S. productivity. Most of Japan's techniques were adopted on American advice and were just followed better than they ever were here, the panelists said.

In Japan, it is recognized that "the specifications for designs are more important than the production process," said Dr. Genichi Taguchi, head of the Japanese Academy of Quality that advises industry and government on quality control techniques. While Japanese educational institutions do not stress early attention to quality more than U.S. colleges do, industrial management focuses on design quality far more than U.S. business, he said.

The new complexity of modern products has outdated old production-line quality control methods, said Ed Fuchs, director of quality assurance at the Bell Laboratories. But the adversary tradition between U.S. management and its labor force will make it hard to set up new methods to assure high standards at every stage in the process, he said.