The Census Bureau's method of counting federal personnel stationed overseas has cost Massachusetts a House seat and added a seat for the state of Washington.

Rep. Thomas C. Sawyer (D-Ohio), chairman of the Post Office and Civil Service subcommittee on census and population, informed members of the Massachusetts delegation Thursday that, based on information he had received, Massachusetts would have retained the seat it lost in the 1990 census if military and federal employees abroad had not been included in the reapportionment tallies.

Military personnel overseas were included in the decennial population counts for reapportionment for the second time this year. The Census Bureau first counted military personnel overseas in 1970, when troops were stationed in Vietnam.

Rep. Joe Moakley (D-Mass.), whose South Boston district could be affected by redistricting, said he was looking into the possibility of a legal challenge or some other effort to regain the lost seat.

"I'm studying the situation very carefully in light of the fact that this is only the second time in history this has been done," he said.

Election Data Services, a Washington, D.C., consulting firm that calculated the effect of the counting method, reported that without federal personnel overseas, Massachusetts would have retained its 11th House seat by 1,033 people and that Washington state would have missed gaining its ninth seat by 836 people.

The Census Bureau counted 922,819 federal employees and their families overseas in 1990.

The 435 House seats are distributed by a complex formula based on population. In many reapportionments, states have won the 435th congressional seat by fewer than 1,000 residents.

The Washington state delegation had been aware that it had won an additional seat by a narrow margin and that court challenges could mean losing it. The new information on the overseas count adds another potential source of challenge, said Kraig Naasz, a spokesman for Sen. Slade Gorton (R-Wash.).

"We knew we might be forced to relinquish that seat if for some reason the Census Bureau might be forced to redetermine the population counts," Naasz said.

The Commerce Department, which oversees the Census Bureau, initially announced it would count overseas military personnel as residents at the last U.S. military base where they had served at least six months. That method of counting would have boosted the populations of states with large military bases, including California, Texas, Georgia and Virginia.

The House passed legislation in June requiring the Census Bureau to instead assign residency by "home of record," the residence listed at the time the person joined the military. Similar legislation was pending in the Senate when the Commerce Department announced it would use "home of record."

If the Census Bureau had used the "last duty station" method, it is likely that more states would have been affected than under "home of record," according to congressional aides.

Sawyer said yesterday that, given the administration's decision to count military personnel overseas for reapportionment, "the allocation of population according to 'home of record' was the fairest method when weighed against the alternative methods available."

Other Americans living away from home, but within the country, including college students and prisoners, are counted where they are residing on April 1, when the census is taken. The almost 8,000 prisoners at the District's Lorton facility, for example, were considered by the Census Bureau to be Virginia residents.

The District government has announced it will file a legal challenge to have those prisoners counted as D.C. residents because the facility is owned by the District.