Big-city mayors voiced dismay yesterday at the federal government's decision not to statistically adjust the 1990 census to compensate for overlooked minorities.

"Absolutely ludicrous," said Mayor Dan Young of Santa Ana, Calif.

"I'm a little flabbergasted," said Mayor John Rousakis of Savannah, Ga. "It seems that when the feds want to find you, they find you. Listening to this, when they want to lose you they lose you."

The mayors were angered by the Commerce Department's decision last October to reject the recommendation of some Census Bureau staff and the mayors that they use statistical procedures to compensate for what experts say is an inevitable omission of some black, Hispanic, Asian and other minorities in the 1990 census.

Those census figures will be used as a basis for congressional and state legislative reapportionment and, most importantly for mayors, in formulas for distribution of billions of dollars in federal aid.

Mayors and some minority advocacy groups say that blacks, Hispanics, Asians and other ethnic minorities are underrepresented for a variety of reasons. Among them are fear that they will disclose violations of housing codes, welfare restrictions or illegal alien status.

The U.S. Conference of Mayors, holding its winter meeting, is urging Congress to order the Census Bureau to compensate for missed minorities in the 1990 census. Mayors questioned a Census Bureau official about the decision in a meeting of the conference's urban economic policy committee.

Peter A. Bounpane, assistant director of the bureau, acknowledged that some people will be missed but said the bureau has taken steps to ensure the best count possible. He cited the hiring of black and Hispanic advertising firms and special appeals in 30 foreign languages.

But Barbara A. Bailar, a statistician and high-ranking Census Bureau official who resigned over the dispute, said the decision not to adjust the figures was made despite recommendations by staff and outside statisticians that they had a sound method for making the adjustment.

"The reasons were dressed up in technical language, but it was purely a political decision that these invisible Americans will not be counted in 1990," said Bailar, former associate director for statistical standards and methodology.

She said that since 1980 the bureau had developed a technique for estimating the undercount and that the methods had been successfully tested in several cities.

Terri Ann Lowenthal, staff director of the House subcommittee on census and population, said the panel would make a decision soon on legislation ordering the adjustment.

Republican Mayor Robert Isaac of Colorado Springs, chairman of the mayors' committee, said the conference's executive committee would consider what further steps it could take to lobby Congress.

Young, the mayor of Santa Ana, estimated his city's population, put at 203,000 in 1980, was underestimated by a third.