Buy low, sell high is still the best free advice federal workers can get as they plan 1991 investments in their tax-deferred thrift savings plan. Employees have until Jan. 31 to sign up for, or change options in, the $7.7 billion fund, which is growing by $12 million each workday.

This year for the first time there are no investment restrictions. Employees can put some or all of their past and future contributions into any of the fund options: The G-fund, short-term Treasury securities; the C-fund, invested in the Wells Fargo Equity Index (stock) Fund; and the F-fund, invested in the Wells Fargo Bond Index Fund.

New data from the thrift board confirms that the stock and bond markets can be roller coasters and are not for timid or short-term investors.

For example, can you handle a 31 percent gain one year and a 3 percent loss the next?

People in the Federal Employees Retirement System can put up to 10 percent of their pay in the plan. Those investing 5 percent or more get a 5 percent government contribution as well.

Those in the old Civil Service Retirement System can invest 5 percent. The savings plan is an important part of the CSRS retirement package, and an absolutely critical component for those (mostly post-1983 hires) under the FERS pension plan.

The G-fund started in 1987. The C- and F-funds were made available on a limited basis in 1988. But the Federal Retirement Thrift Investment Board has estimated how the funds would have performed if workers could have invested in them since 1980.

For the 1980-89 period, it says, the C-fund would have produced a 17.46 percent annual rate of return; the F-fund a 12.43 percent return, and the Treasury G-fund, an 11.04 percent annual rate of return.

But that doesn't reflect the moves of the markets, or the steady, safe return of the Treasury option. Example: In 1989, the stock fund racked up a 31.03 percent gain, but in 1990 it lost 3.15 percent.

Here are the actual performance levels of the three funds. The 1987 G-fund figure reflects a nine-month year because investments started in April. The stock and bond funds were made available in late January 1988 so rates for that year reflect an 11-month period.

The track records:

Treasury Fund: The G-fund's 1987 return (for April through December) was 6.42 percent. In both 1988 and 1989, it was 8.81 percent. In 1990, it was 8.9 percent.

Stock Fund: The C-fund option became available in 1988. For the period from February through December, its return was 12.06 percent. In 1989, it was 31.03 percent. In 1990, it lost 3.15 percent.

Bond Fund: The F-fund also became available in 1988. For the period from February through December, it was 3.7 percent. In 1989, the rate was 13.89 percent. Last year the bond fund had an 8 percent rate of return.

Most financial advisers urge would-be investors to adopt a long-term strategy. That is why the thrift plan summary book employees got in November has past performance charts. Although they don't predict future earnings or losses, they show people where the funds have been.