Most of the black families moving from Washington to Prince George's County have incomes well above the District's average, according to a study refuting a widely held belief that the suburb has been a "dumping ground" for poor urban blacks.

The new analysis of U.S. Census Bureau data found that the median income of 8,900 black households who moved fromthe city to Prince George's in 1977 was $19,010 -- about $6,000 above the average Washington household for all households in the suburban Maryland county.

"There's been an awful lot of mythology about who is moving into Prince George's," said demographer George Grier, who helped prepare the data that provides the information on the large exodus of blacks from Washington since 1970.

"Unquestionably, Prince George's is getting some poor households from Washington," Grier added, "but they are a small minority compared to the middle-class families they are attracting."

In fact, Grier said, in 1977 Prince George's received twice as many low-income white households [earning less than $10,000] from places outside the Washington area than blacks with low incomes from Washington.

Grier said the data indicates that the black households moving from Washington to Prince George's are mostly "young, upwardly mobile and middle-income." Most have probably moved, he said, for the same reasons that earlier drew whites to the suburbs -- the quest for more space, better schools and better neighborhoods.

Meanwhile, Grier said, poorer blacks who are being displaced by renovation on Capitol Hill and other central-city neighborhoods, apparently are moving to relatively low-cost, low-quality housing farther out in the city, and not to Prince George's or other suburbs.

Although there is no direct evidence available on where poor people from renovated neighborhoods are moving, the Census Bureau data indicates that Washington had a larger share of the area's poor households in 1977 than it did in 1970.

In 1977, the District had 45.5 percent of all area households earning less than $10,000 a year, compared to 4.15 percent of such households seven years earlier.

"The idea that the black poor are being forced out of the city and into the suburbs is a very popular notion," Grier said. "But it simply isn't supported by the data."

The statistics in the new report by Grier and his wife Eunice, derived from a survey of 15,000 housing units in the Washington area conducted by the Census Bureau in 1977 and early 1978. The census published much of its data last month in 498 pages of complicated tables that focus mainly on housing conditions. In addition, the Griers extracted figures by computer from a magnetic tape file of the survey data provided by the Census Bureau.

Most of the income data in the report, issues by the Greater Washington Research Center, is given as a median, which marks the midpoint in any group of figures. Half of those in a particular area earned more than the median, and half earned less.

According to the census data median household income in Prince George's climbed by 76.2 percent from 1970 to 1977. The rise was almost exactly the same as the increase for the whole metropolitan area and kept Prince George's just slightly above the areawide average. During the same years, blacks increased from 14 percent to 31 percent of Prince George's population.

Grier said the data indicates that, despite beliefs expressed by some white suburbanities, the upsurge of blacks in Prince George's has not led to more economic problems for the county or a drop in its relative position in the area.

The black families going to Prince George's from Washington have a median income about 7 percent less than the countywide average of $20,420, Grier said, but the difference has little significance because the newcomers are relatively young. More than 70 percent of them have household heads under age 35.

"They probably have more income potential," Grier said, "than the people already living in Prince George's."

During that same period, 1970-77, themedian household income in Washington rose more slowly than average.

Grier noted that the income gap between the city and its suburbs stopped widening after 1974 as more upper-income whites moved into the District. But it apparently hasn't closed because the city is losing middle-income black families while keeping most blacks who are poor. The city's population was 71 percent black in 1977, about what it had been in 1970. Most low-income people, Grier said, live in apartments, and despite inflation and the loss of some rental housing, there are still far more low-rent units in the city.

Even though prices of houses have risen more rapidly in the city than the suburbs since 1974, rents have climbed much less because of rent control, subsidized units, and the deterioration of some buildings and neighborhoods.

According to the Census data, the District had 86,600 units with monthly rents below $175 in 1977, compared to just 30,200 such units throughout the suburbs.

The poorest families, who can afford only low-rent units, moved much less than those who are middle-income and have more choice.

The Griers report also makes the following points:

In the 1977 sample, 81 percent of the 11,000 black households that moved from Washington to the suburbs went to Prince GeorgeS county. Another 14 percent moved to Montgomery County, and just 5 percent to the Virginia suburbs.

Prince George's now has about three-fifths of all suburban black households. Yet, the increase has been about the same in both Montgomery and Prince George's -- nearly three times more than in 1970. Blacks living in the Virginia suburbs have doubled since 1970, though the numbers involved are relatively small.

During the first half of the 1970s, 55 percent of area families moved at least once. Those who moved in 1977 accounted for nearly 22 percent of the 1,052,100 households in the area.

Of the families living in the Washington area in late 1974 who had moved since 1970, about 40 percent had come from outside the area, and 36 percent had moved within the same county or city. Of the remainer, almost 11 percent moved from one suburb to another, 6 percent from the District to the suburbs, and just 1.3 percent from the suburbs to the District. The previous residence for 5 percent of them was unknown.

Of the new households that moved into the District during the first half of the 1970s, 83 percent came from outside the metropolitan area, and just 17 percent moved in from the suburbs.

Overall, the area households that move tend to be young, small, and middle-income. In 1977, about two-thirds of them had household heads under age 35, some 60 percent had only one or two persons, and 46 percent had incomes ranging from $15,000 to $35,000 a year.

Throughout the area 70 percent of those moving were white, 26 percent black, and 4 percent of other races, mostly Asians.

Despite a populaiton loss of almost 100,000 since 1970, Washington had a 15,000 increase in the number of female-headed households. By 1977, women accounted for 45 percent of all household heads in the District, compared to just 22 percent of all household heads in the suburbs. In both the city and the suburbs, about half the female household heads were woman living alone. The other half lived with someone else, usually their children.