America's ability to compete against major trading partners has eroded dramatically and dangerously in the last 15 years, according to a new index from the Council on Competitiveness.

The council, a non-partisan research and education organizaiton, unveiled Thursday its Competitiveness Index showing the U.S. standard of living since 1972 has risen only one-seventh as fast as that of Japan and one-half as fast as that of West Germany.The index also showed the United States lagging behind other major industrial democracies in trade, productivity and investment, three other key components of a competitive nation. The index measures America's performance in all four categories against its past performance and against foreign competitors.

Among the findings:

- The U.S. standard of living grew more slowly during the past year than any other of the seven major industrial democracies.

- The standard of living per American worker was $28,400 in 1987, well below West Germany's $34,400.

- The U.S. share of world exports, despite an export boom in the past year, was only 10 percent in 1987 compared to 12 percent in 1972.

- Since 1972, Japan's productivity has risen eight times faster than that of the United States, while West Germany's was three times faster.

- Long-term U.S. investment in education, capital equipment and research and development has not matched investment spending by trading partners during the past 15 years, and U.S. investment levels in 1986 were lower than in 1972.

The Competitive Index, which will be updated every six months, fills a gap of knowledge and understanding between anecdotal evidence and government economic statistics that can be misleading or incomplete, the council asserted.

"We are awash in economic statistics . . . that don't tell the story," said Michael Porter, the Harvard University economist who helped prepare the index.