Ihave in front of me a check -- or, more accurately, the facsimile of a check -- made out to the working women of America and issued on the account of the National Federation of Business and Professional Women's Clubs. The check is for the pay period beginning Sept. 5 and ending Dec. 31. The paycheck amount is $0.00.

The pay stub puts the message another way: "Working women in this country average only 68 cents for every dollar a man makes. If a working woman's paycheck was equal to that of a man's, her salary would last only 68 percent of the year.

"September 4th is the last payday for the average working woman compared to a man. Her salary would have run out. From September 5-December 31, 1987, working women in this country really receive no pay for the work they do, while men still get paid."

That may be one of the most provocative ways developed for putting the salary gap in perspective. It is one thing to hear government figures about how women earn 68 cents for every dollar that men earn, and it's quite another to think of working for nearly a third of the year for nothing. Perhaps we could start writing off our parking and transportation costs on our income taxes under the section where you deduct the costs of volunteer work.

The Census Bureau announced shortly before Labor Day that the gap between men's and women's earnings had narrowed to 68 cents on the dollar, based on hourly wages. Wages are measured in different ways by different agencies of the federal government, such as the Labor Department and the Census Bureau, and the hourly and weekly earnings ratios have generally shown women to be doing better than have the annual earnings measurements.

The Census Bureau's September announcement that women's wages had increased to 68 percent of men's wages in 1986 from 62 percent in 1979 received considerable attention. The news appeared to be one of the biggest breakthroughs in the wage gap, which has remained relatively constant despite the dramatic increase in the number of working women and despite the fact that half of professionals are women.

The National Committee on Pay Equity has analyzed the Census Bureau's report, however, and concluded that the news is not as good as it originally appeared to be. The group has been in the forefront of efforts to use comparable worth as a method of improving the wages of working women in the United States. It prepared a briefing paper clarifying some of the confusion that surrounds the salary gap and the various figures that are often cited in discussion of it. The paper, using census and other government data, was prepared with the assistance of Heidi Hartmann, director of the Institute for Women's Policy Research and former study director on women's employment issues at the National Academy of Sciences.

Based on annual earnings, the female-male earnings ratio rose from 59.7 percent in 1979 to 64.3 percent in 1986 for full-time workers. Three-quarters of the improvement in women's wages versus men's wages came from growth in women's earnings, but a quarter represented a drop in men's earnings, largely because of declining employment in higher-paying manufacturing jobs. The pay equity group traced the annual earnings ratios and found that the figure has hovered around 60 percent since 1955, dropping to a low of 57 percent in 1973 and 1974 and rising to highs of 64 percent in 1955, 1957, 1983 and 1984. From 1975 to 1981, it was 59 or 60 percent. After 1981, it climbed steadily to a peak of 64.6 in 1985.

"This slow climb in the past few years does represent progress for women," the report found, "but it is important to note that the ratio has been nearly as high several times in the past." Whether the ratio continues to improve depends on a number of factors, including the strength of the economy and the willingness of employers to eliminate wage disparities based on sex.

As has been the case in the past, young women are doing better in the female-male earnings ratio than women over 45. Education and training are contributing to their improved earnings, says Hartmann. "College majors are more similar to men's than they were 20 years ago. There are fewer education majors among women, and more business and science majors."

What is not known, she says, is whether this younger group of women will be able to command a higher earnings ratio relative to men than their mothers have been able to manage. What is known, however, is that for women taken as a whole, the salary gap -- no matter which numbers you use -- remains an intransigent economic barrier. Chances are that from Labor Day on they're working for free.