A massive new Census Bureau study of the nation's poor shows not only that the proportion of Americans living in poverty has remained almost constant in the two most recent years studied, but that certain groups are far more likely to be poor than others.

They are:

Black or Hispanic Americans.

Persons who never finished high school.

Women, especially elderly widows.

Children living in female-headed homes.

Persons who live in the South, a rural area, or a decaying Northern city -- almost anywhere but the suburbs.

Conversely, the statistics, based on surveys taken in March 1979, show that well-educated, working-age whites from suburban mom-and-pop families are least likely to be poor.

In the survey, a family of four was considered poor if its 1978 cash income was less than $6,662; the figure was proportionately lower for smaller families and higher for larger ones.

The survey showed that 24,497,000 people, or 11.4 percent of the nation's population, fell below the poverty cutoff, a tiny reduction from the previous year, when the figure was 11.6 percent.

But the 11.4 percent figure masked tremendous differences among different population groups.

For example, among whites, the percent under the proverty threshold was 8.7 percent, but among Hispanics it was 21.6 percent, and blacks 30.6 percent. 8

The survey showed that low-income people usually had an inferior education: two-thirds of the poor over age 13 lacked a high school diploma, while only one-third of the overall population above 13 failed to finish high school.

The poverty ratio was higher among women than among men: about 14.5 million of the poor were women, only 10 million were men. A big clump of these were over-65 women (2.3 million), a large proportion of them, widows.

Children also made up a big portion of the poor: about 9.7 million. They came largely from female-headed families, which made up half of all the poverty families.

In general, only 5.2 percent of households with both a husband and wife fell below the poverty line, but nearly one-third of the households headed by a woman without a father present were in poverty -- and for black female-headed units, the figure was one-half.

The figures showed that the region of the nation with the most poverty is still the South -- where 14.7 percent of the population was classified as poor, compared with 10.4 percent in the Northeast, 10 percent in the West and 9.1 percent in the North Central states.

By type of neighborhood, only about 6.8 percent of suburbanites are poor, but for central cities the figure is 15.4 percent and for rural areas, 13.5 percent.

Among the nation's largest cities, Philadelphia, Chicago and New York have the highest proportion of center-city residents in poverty -- about one person in five. Washington is about the same, with an 18 percent poverty ratio. But when the metropolitan area, including the more affluent suburbs, is considered, the Washington area drops to only a 6.1 percent poverty ratio. New York, with a 13.6 percent poverty ratio for the whole metropolitan area, leads the nation in poor people.

The figure also show that poor families get about two-fifths of their income from jobs, and most of the rest from welfare and Social Security payments.

About 14 million people just above the povery line would drop below it if welfare and Social Security payments they receive were eliminated.