WHEN THE Census Bureau reported the other day that 13 percent of the American people still live in poverty, some people, as usual, dismissed the numbers as an exaggeration. The reason is that the Census Bureau does not include all kinds of government aid to the poor as part of these people's income. So while an urban family of four may have less than $8,414 in annual income--a poverty level income--that same family could be receiving food stamps and Medicaid and other forms of so-called "in kind" income.

How, then, is anyone to judge just how many poor people live in the United States? Martin Anderson, the president's economic adviser, wrote in his 1978 book "Welfare" that "the war on poverty that began in 1964 has been won. The growth of jobs and income in the private economy combined with an explosive increase in spending for welfare and income transfer programs has virtually eliminated all poverty in the United States." The Congressional Budget Office did not find that all poverty had been eliminated when it reviewed the subject in 1977. But the CBO did say that, by its reckoning, if all transfer payments and income were included, the nation's poor would be about 6 percent of the population; at the time, the Census Bureau put it at 12 percent.

In all the controversy over the statistics of poverty, the key point is often neglected: the difficulty of calculating the amount of American poverty does not disguise the fact that it still exists. And in the last year, no matter whose figures may be employed, the rate of poverty has gone up. On the Census Bureau's scale, the jump has been from 11.7 percent in 1979 to 13 percent in 1980. The largest components of that increase were among people of Spanish-speaking origin, people living on farms, and female- headed households. Statistical arguments about poverty should not obscure the truth that poverty is growing in the United States.