WASHINGTON (AP) _ Millions of Americans move into and out of poverty each year, according to a new Census Bureau report that also found a base group of chronically poor.

Relatively small changes in the total number of people living in poverty from year to year mask the fact that millions change poverty status annually, according to the study, ``Dynamics of Economic Well-Bring,'' released Thursday.

The income analysis found that 18.8 million people who were poor in 1990 remained poor in 1991. But 5.1 million people moved out of poverty between the two years and 6.2 million became poor.

The Census Bureau does not study the causes of people moving into or out of poverty, said analyst Martina Shea, but her report did note that 1990 and 1991 were years that the economy was in recession.

Looking at 1990, the report found the median duration of poverty was 4 months. Being poor lasted longer for blacks, 5.8 months, compared with 3.9 for whites. People who had not finished high school were poor for 5.3 months; those with a disability, 4.9 months, and rural residents, 4.7 months.

The chronically poor, who were in poverty throughout both years, were 60.5 percent white and 34.4 percent black. A big share were under age 18, 43.9 percent, while 46 percent were aged 18 to 64 and 10.2 percent were 65 and older.