The days when Uncle Sam employed an army of clerks in green eyeshades is long gone, according to a new report by the Congressional Budget Office. CBO's statistical snapshot of the federal establishment shows one of the nation's best-educated and most important corporations with a productivity rate that would be the envy of many private firms.

CBO's study says that 70 percent of the federal work force is directly involved in national defense or delivery of health or veterans services or the mail.

Partly because of productivity, CBO says, the federal establishment -- which has just under 3 million part-time and full-time civilian workers -- has 270,000 fewer employees than it would have otherwise needed just to keep pace with demands caused by population growth during the last decade.

Outside of the three largest federal operations -- the Defense Department, the U.S. Postal Service and the Veterans Administration -- the CBO report to Congress shows that government employment has been remarkably stable since 1977.

In the past 10 years, the number of nonpostal federal workers with bachelor's degrees or higher has risen from 25 percent to 31 percent, a much higher figure than the private sector as a whole. About 88 percent of all professionals and 46 percent of the workers in administrative jobs have degrees, CBO says.

While the number of well-educated professionals has increased, CBO says that average length of service for nonpostal workers dropped from 14.1 years in 1976 to 13.5 years as of 1985. Some of the decline is attributed to the crush of new hires during the recent defense buildup, the fact that fewer new workers are veterans (who tend to stay in government longer and whose military time counts toward civilian federal time), and the growing number of women -- now 40 percent of the nonpostal work force -- who have greater job turnover because of marriage and children.

In addition to clerical jobs, which always have high turnover, CBO warns that losses in the upper ranks, the Senior Executive Service, should be watched to see if they foretell "some long-term trend" that could cost the government its best bosses in the future. The CBO study was completed before the most recent federal pay raise, which Congress refused to give to most members of the SES.

CBO's statistical profile of the federal service shows two things: that the government is very big (and therefore a prime target for budget cutters) and that it is also hard to cut, because it is big in areas that provide direct services to the public.

For example, CBO says that 200,000 federal workers provide health and nursing home care to war veterans; 185,000 collect taxes and support programs, such as employee pay and benefits, and manage the government's complex finances. Four departments -- Defense, the National Aeronautics and Space Administration, Health and Human Services and Commerce -- employ most of the 165,000 federal workers who are involved in research and information activities that have important constituencies.

About one of every four federal workers is in the U.S. Postal Service.

It may well be that the government is much too big, but the question is, what would you eliminate?