Unemployed American workers in May had been without jobs for a total of 228.3 million weeks, the highest monthly total since at least 1948, according to a new study by a Democratic economist for the Joint Economic Committee of Congress.

The study, released yesterday by Rep. Lee H. Hamilton (D-Ind.), provides a new statistic computed by multiplying two numbers released every month by the Bureau of Labor Statistics: the total of unemployed people on a seasonally adjusted basis and the mean duration of their joblessness.

In May, the unemployment rate dropped from 10.2 percent to 10.1 percent, continuing a downward trend that started in January, and 11.2 million people were out of jobs, also a slight decrease.

The mean duration of unemployment, however, increased slightly from 19 weeks in April to 20.4 weeks in May. That means half of those unemployed had been without jobs for less than 20.4 weeks and half for more than 20.4 weeks. The result of multiplying the mean duration times the 11.2 million unemployed is 228.3 million total weeks of unemployment.

"Traditional measures such as the unemployment rate and the number unemployed fail to consider the length of the period of joblessness," Hamilton said. "Clearly someone out of work for one year is subject to much greater hardship than someone out of work for one week."

Paul Manchester, an economist and author of the JEC study, wrote that it shows "a current labor market picture much worse than that shown by the conventional measures, and one much more severe than the situation in the 1973-75 recession . . . . "

Martin Ziegler of the Bureau of Labor Statistics, said yesterday that "One of the problems with the statistic is that as your recession comes to an end, the duration of unemployment increases" because fewer layoffs occur to drive down the mean.

Another factor possibly affecting the statistic would be the growth in the total work force: when it grows, the number of unemployed people would be higher although the percentage of unemployed people might be lower.

There also has been some speculation that continued extensions in the availability of unemployment benefits may have kept some people on the unemployment list.