It seems that over 300 deaths must have been covered up at the time, which would have made 784 a significant undercount of prisoner deaths to date.

The critics charged that the change would result in a significant undercount of the number of black and Hispanic viewers, citing results of a test this winter that showed that the ratings for some series popular with those audiences fell sharply.

But as fears grow that a significant undercount is all but assured, Bureau Director Barbara Everitt Bryant played down the difficulties in a report to the subcommittee last Monday.

Thomas Jefferson complained that the first census, in 1790, contained a significant undercount, the Chief Justice said.

"We had to do some extrapolating ourselves, in a hasty fashion, hoping we were getting ones with significant undercounts," Mr. Ceasar said.

It should be noted however, that there is a significant undercount, as many of the biggest Indian bands regularly refuse to participate in Canadian censuses for political reasons regarding the question of aboriginal sovereignty.

At yesterday's hearing, Mayor Koch and others said they feared the new law would discourage illegal aliens from being counted in the 1990 census and cause a significant undercount of New York City's population.

But behind the hoopla lies the fear of a significant undercount that would lead to losses in congressional representation and Federal aid.

The census was also noted for a significant undercount of the population.

That is a blow against political fairness because states with immigrant, minority and low-income populations have suffered significant undercounts under traditional census-taking methods, making them likely to be shortchanged on seats in Congress.