A decision by the Office of Management and Budget to restructure its Statistical Policy Branch has raised new complaints that the Reagan administration is cutting federal statistical programs too deeply.

News of a reorganization of the policy unit, which oversees and coordinates government statistical studies, recently leaked out of OMB after the unit's employes reportedly were told they would be reassigned.

A few hours after that meeting, Rep. Henry S. Reuss (D-Wis.), chairman of the Joint Economic Committee, sent OMB Director David A. Stockman a telegram asking why the office was being "abolished."

"OMB's action is a further step backward for intelligent and informed policy making and will only confirm those who have dark fears of a massive statistical cover-up in the making," Reuss wrote.

Rep. Paul Simon (D-Ill.), chairman of a House Budget Committee task force studying indexing and entitlements, has also written Stockman and is circulating a "Dear Colleague" letter, protesting the "dissolution" of the office.

OMB spokesman Edwin L. Dale Jr. acknowledged that the statistical unit will be restructured as part of a larger reorganization of the Office of Information and Regulatory Affairs. Dale said no final decision has been made, but he said the unit will continue performing its statistical duties. "We expect to issue a full explanation in the near future about the changes," said Dale.

The size and power of the statistical unit, which has been around since the days of Franklin D. Roosevelt, has been steadily downgraded in recent years. The office, which had as many as 70 staff members some 30 years ago, now has the equivalent of 9 1/2 full-time positions.

Privately, OMB officials said one reason the unit is being restructured is because it has been ineffective, a charge with which many statisticians agree. Even so, many statisticians say the government needs a central statistics office.

The federal government has more than 70 statistical programs run by many different departments, explained Courtenay Slater, former chief economist at the Commerce Department. Without one coordinating unit, she said, the quality and timeliness of major economic data would be threatened.

Nobel prize winner Wassily Leontief also has called for one statistical office. He told a congressional committee that "the United States is the only advanced industrialized country that still does not possess a real, central statistical office . . . "

The reorganization also comes at a time when many statisticians say the quality of government statistics is in jeopardy.

The government's two biggest statistics collectors, the Census Bureau and the Bureau of Labor Statistics, have been forced to slice programs because of budget cuts. BLS has ended 19 studies, stopped all travel, and failed to replace 100 employes who quit or retired this year. The agency has asked for an emergency $5.6 million appropriation which, it says, it needs "to maintain the accuracy" of such key economic indicators as the Consumer Price Index.

The Census Bureau has dropped a host of studies and decided not to fund a new five-year census in 1985. It has also canceled a special sample in its 1983 census of agriculture and stopped collecting farm data from Guam, the Virgin Islands and Puerto Rico.

Many of the programs being trimmed helped the government monitor how its programs were being used. Others helped policy makers predict economic trends.

"A million dollars saved today through short-sighted reductions in the budgets for statistical programs could lead to erroneous decisions that would cost the private and public sectors billions of dollars over the long run," John J. Casson, chief economist for American Express, warned at a congressional subcommittee hearing in March.

Witnesses for the U.S. Commission on Civil Rights and the Children's Defense Fund told the subcommittee that cuts had deprived them vital data about minorities and the handicapped. A spokesman for the AFL-CIO accused the BLS of making "anti-labor cuts," because, he said, the agency canceled several programs that provide information for unions to use in negotiations.

OMB said the cuts were not political. The programs with the fewest users were cut, Dale said, as part of the government's belt-tightening.

Earlier this month, the Congressional Research Service released a 53-page study that provoked even more concern among statistics watchers. The report said that budgets for statistical programs would decrease by another 5.1 percent under the proposed fiscal 1983 budget.

OMB insists the budgets for statistical programs either will hold steady or increase in fiscal 1983 and claims the flow of crucial economic data will not be impaired.

Daniel B. Levine, a former deputy director of the Census Bureau who left in January to join private industry, agreed that despite the cuts, the government's most important data is as good or better than information collected in the past.

But, Levine said, the real impact of the cuts could be felt "later down the road." Young and imaginative statisticians are leaving the government, he said, and government managers are so busy looking for programs to cut that they don't have time to improve data collection.