Last July, Donald L. Amen sat in the basement of the Agriculture Department building with a great sheaf of computer printouts and a slight inclination to transfer from the government's old Civil Service Retirement System to the one Congress passed last year.

As a 41-year-old staff specialist in employe relations, Amen was looking forward to years of federal employment and had the means and the self-discipline to take full advantage of the tax-deferred savings plan that is the cornerstone of the new Federal Employees Retirement System (FERS).

But, like the overwhelming majority of federal workers hired since Dec. 31, 1983, Amen has decided to stick with the Civil Service Retirement System because it provides better benefits for those who retire.

"FERS might be better for me," he said, "but if I have a chance to retire early, I don't want to feel pressure to keep working just for the retirement system." Amen is one of two million federal workers who must make an irrevocable decision before January to stay in the old system forever or transfer to FERS.

Only 3 percent have decided to switch, far fewer than the 40 percent expected to change after the bill was passed with one of the most generous savings plans in the nation.

Various proposals to extend the deadline and to drop some of the requirements that might restrict savings plan participation by higher-income workers are pending before Congress, and the outcome is unclear.

In the meantime, many federal workers are probably making a big retirement mistake.

One reason apparently lies in what one federal worker described as the "Kafkaesque quality of what the government is trying to get the federal employe to do."

First, the law is extremely complicated. Second, it is uncertain whether it will be changed after the deadline. Third, Congress could very well change the benefits of either or both plans in the future.

The number of people choosing FERS has been "a dismal failure," the federal worker said, "because the decision is based on 10 to 15 different choices that the greatest economist in the country couldn't decide."

These include estimating what will happen to the economy, the stock market, bonds, government securities, inflation and the employe's own career.

"The decision is so complicated," he said, "that you need a computer. But there are six different computer programs, and you have to read Consumer Checkbook {a Washington consumer publication} to see which computer program to choose."

His judgment is harsh, but Congress and the administration clearly miscalculated the reaction to FERS, which was passed by Congress in 1986 after two years of study and compromise. It automatically covers everyone hired since Dec. 31, 1983, and has three parts: Social Security, a small annuity and a tax-deferred savings plan.

Workers hired before that date must choose whether to stay in the Civil Service Retirement System or switch to FERS.

Until now, federal workers haven't been covered by Social Security, and if they left the government before retirement age they got back only what they paid into the Civil Service Retirement System from their own salaries, without interest. As a result, the old system has tied some workers to hated or dead-end jobs with "golden handcuffs."

For this reason, FERS is considered a better system for many, because it is "portable" -- if employes leave the government for private industry or academia, they take along their Social Security credit.

FERS also attempts to boost the average federal retirement age by a combination of penalties against early retirement and incentives to work longer. Under CSRS, workers could retire at age 55 after 30 years of service and receive full benefits. Under FERS, the retirement age is gradually moved up to age 57 after 30 years of service, and workers get a slightly richer pension if they wait until 62 to retire.

The tax-deferred savings plan is the part of FERS that is new to the government and has received the most attention. It allows civil servants to make individual decisions about how much to save and where it should be invested.