Students of that gloomy science, Economics 101, usually are given a simple image to portray the distribution of income in the United States.

It's like the Eiffel Tower, they're told, with nearly everyone in the country in the first five or 10 feet, and the highest earner 984 feet up, at the very top. Or sometimes the marching band analogy is used, where if each minute represents $10,000 of income, 90 percent of the population would pass by in the first 30 minutes, but the individual raking in the most cash wouldn't show for 24 hours.

"Those are simple income lines, and give the impression that there are just a few crazies out there who make a real lot of money, with everyone else bunched tightly together," says Stephen Rose, a budget analyst for the Washington State Senate.

In the mid-1970s, the then-professor of economics at the University of Maryland in Baltimore decided he wanted to give people a better sense of where they are -- "a freeze-frame of American society." The result was The American Profile Poster (Pantheon, $5.95), which recently appeared in its third edition.

The poster looks vaguely like either a pregnant woman, the state of Idaho, or, as Rose says, "a man with quite a tummy." It has six variables -- sex, occupation, household status (husband-wife couples, single men, single women, singles with dependents) and race, which are divided by household income and by wealth.

While the poster conveys a good deal of information, it does so in a simple way. To take a random example, a viewer can see at a glance that while more than 2 million retired white men and women have incomes above $30,000, the number of nonwhite retired people at that level isn't even large enough to register.

In making clear who earns what, the poster shows the diversity of American society, and the relative proportion contributed by each group. But the picture presented isn't the expected one.

While a guest speaker at a program for gifted and talented students in Baltimore, Rose asked the teen-agers to write two numbers down: their family income, and what their percentile was.

"I generally got an answer of $80,000 family income and in the 70th percentile -- meaning they thought 30 percent of families made more than them and 70 percent made less. But $80,000 is really the top 3 percent of families. If these kids were in the 70th percentile, their parents would only be making about $36,000. They were really off on where they stood in American society."

He also spoke at an educational forum for a church group in the same city, where the audience told him they thought most people "topped out" at $30,000. "When asked where that income fits into American society, they said it's the 85 percentile, meaning only 15 percent of the population makes more. But actually 40 percent of the population makes more."

According to Rose, this is a widespread cultural belief: the upper middle class thinks it's further down and the lower middle class further up the scale than either actually is.

"Middle class is such a widely used term, you could use it to say one thing when you really mean something else. For example, the 1981 tax cut was advertised as helping the middle class, but those earning $15,000 got a break of about $1, while those making $80,000 got several thousand dollars."

Part of the confusion arises in an area near the middle of the poster -- those earning between $24,000 and $36,000. Since this is the top of the blue collar and the middle bottom of the professional/managerial, the two groups can think of themselves as similar, part of the great American middle class.

"It's an illusion," says Rose. "The professionals are on their way up, while the blue collar workers have often reached their maximum earnings. The notion of one homogenized bunch is nice, but it's not the reality."

If the poster is where we're at, the changing economic figures since its first edition in 1978 provide a glimpse of where we may be heading. Simply put, the "man with quite a tummy" has seen his stomach go even lower. At the same time, his shoulder muscles have improved.

In other words, some of the middle class are doing better, but even more are doing worse. Of those earning between $19,000 and $47,000 in 1978, 2.8 percent are now over the upper line, while almost twice that amount -- 5.2 percent -- have fallen below the bottom figure.

"There are two income trends in the post-war period," Rose says. "From 1947 to '69, there's greater income equality, meaning the people at the bottom did relatively well. But from '69 to the present, inequality increases."

Reasons for this include a weaker economy, which causes a loss of high-paying blue collar jobs and more union give-backs. Social explanations include the increasing divorce rate, which has swelled the ranks of single women who are supporting families, and a larger number of young people at the beginning of their careers.

"By and large, only two-earner families are making it upwards," says Rose. "For the rest, the job ladders have clogged."

His prognosis for the future isn't particularly happy, either. If new editions of the poster continues to record a shrinking middle class, he expects to see more tension between the rich and poor.

"When everyone thinks they're in this homogenous, middle-class group, that's a social stabilizer," he says. "If they don't think that, you're going to see the social fabric being tugged at."