(FORTUNE Magazine) – Demographers are the first to admit the inexact nature of their science, which includes projections of U.S. population growth and changes in the country's ethnic mix. But for all the uncertainties, forecasters also warn that managers ignore such predictions at their peril. Says Margaret Regan, a partner at TPF& C, a New York City human resource firm: ''The leading-edge companies take these numbers seriously, not as just crystal ball guesses. They are starting to figure out how they will get the best value out of the country's increasing diversity.'' The future hasn't always turned out as some predicted. For example, the 1990 census showed that in the 1980s the black population increased less than had been forecast, reaching 30 million rather than 31 million. Some prognostications, even though based on how the U.S. has evolved since World War II, are largely speculative, dependent as they are on unknowns like economic growth. The WEFA Group predicts unemployment will decline steadily from today's 6.6% to 4.6% by the year 2003, the lowest it has been since 1969. Other forecasts: -- The Hispanic population will be the fastest-growing segment of the labor force, making up 9.2% of the total in 2000, vs. 7.8% in 1991 (source: U.S. Department of Labor). -- The diversity of the work force will stabilize around 2020 as the birthrate among minorities plateaus (Institute for Workplace Learning). -- Immigration, estimated to reach 685,000 in 1991, will jump to 800,000 in 1993 and remain at that level well into the 21st century. In 1993 immigrants will account for 33% of the population increase (WEFA Group). - K.B.

CHART: NOT AVAILABLE CREDIT: FORTUNE CHART / SOURCES: BUREAU OF THE CENSUS, WOODS & POOLE ECONOMICS, THE WEFA GROUP CAPTION: THE NEW MIX 1991 2021 POPULATION 1991: 117.9 million Americans -- including 54 million women -- have jobs. Births reach four million, matching 1957, the peak baby-boom year. Fastest-growing city: Los Angeles, increasing by 20% from 1980 to more than nine million. Highest per capita income ($24,650): Bridgeport, Connecticut.

1996: Birthrate drops as women on average have 1.8 children, down from two in 1991. Reason: The rush to catch up and have babies is over. In the year 2000, the rate of increase in the size of the work force starts to slow to 0.9% a year from 1.4% in the 1990s.

2001: Life expectancy for men reaches 73.5, for women 80.4, compared with 72.2 and 79.2 in 1991. Medicare payments for the elderly and disabled increase 63% from 1990 to $173.9 billion in 1990 dollars.

2006: Some 15% of the labor force is 55 or older, vs. 12% in 1991. By the year 2010, New England has the highest percentage of elderly among regions, some 14% of its total 16 million population. Of the states, Florida still has the highest proportion of oldsters, who make up nearly 20% of its 18.4 million people.

2011: Baby-boomers start to hit 65 and begin to retire in big numbers. That pushes the total of retirees from 25 million in 1991 to over 33 million, compared with 153 million working Americans. Number of women in labor force: 76.3 million, or 64% of adult female population, up from 57% in 1991.

2020 About 18% of the population is 65 or older, vs. 13% in 1991. Social Security payments reach $558.3 billion in 1991 dollars, almost double 1995's level.On average, each woman will still have 1.8 children, a constant since 1996.