Poverty is a reality for three in 10 Americans, but for most of them, it's short-lived, the Census Bureau reports.

A new study by the bureau, released Monday, takes an unusually deep look at poverty in the United States, using seven measures to paint a picture more complex than any one statistic might suggest.Over a three-year span, 30.3 percent of the population lived below the poverty line for at least two months. But just 5.3 percent of them stayed poor for two full years.

"These statistics portray poverty as a trap door for a few and a revolving door for many," explained the report, which examined data from 1993, 1994 and 1995.

The government considers a three-person family poor if its income is below $13,650; for a four-person family, it's $16,450.

In 1994, on average, 15.4 percent of Americans were poor each month, and about 22 percent - or 55 million people - were poor for at least two months in 1994.

Nearly half of them stayed poor for just two to four months. About 13 percent were poor for more than two years.

On average, people were poor for 41/2 months.

But the rates differ dramatically by race and family structure.

Blacks, Hispanics and children are among the poorest groups in the nation.

But the most likely to be poor were families headed by single mothers: In 1994, nearly half of the female-headed households lived in poverty for at least two months in a row, more than threetimes the poverty rate of married couples.

The difference was even more dramatic among the chronically poor. Single moms were eight times as likely to live in poverty for two years than married couples were to be poor for at least two years.

"You can't understate the role of family structure," said Robert I. Lerman, an economist at the Urban Institute who studies poverty issues.

Children are most likely to be poor, no matter what the measure. And the elderly, once the poorest Americans, are now the least likely to live in poverty, thanks to Social Security and Medicare.

But while senior citizens are least likely to fall into poverty, once there, they are as likely as children to remain poor. That's because they rely on fixed incomes, which are unlikely to go up, Lerman said.

"Where are they going to get an increased income?" he asked.

The report also notes that while blacks and Hispanics have similar poverty rates, blacks were slightly less likely to fall into poverty but slightly more likely to stay there.