Beryl Sprinkel's ''A Back-Door Tax'' {op-ed, July 21} was just the kind of jumbled misstatement of facts and shallow analysis one expects from President Reagan's economic advisers. His attack on the plant-closing and mass-layoff notification bill was especially misguided.

The Labor Department's Task Force on Economic Dislocation published data showing that fewer than half of all dislocated workers get even one day's notice of their termination. How then can Mr. Sprinkel say that ''employee compensation typically includes . . . advance notification of layoff "?

He argued that the Senate's bill requiring 60 days' advance notice of plant closings would cause "higher costs and greater risks in starting or expanding a business," which means "that there would be fewer new firms and fewer new jobs." The facts are otherwise. Canada, whose economy is very much like our own, has had a 60-days'-notice law for more than a decade and has added new jobs to its economy during that time at a higher rate than has the United States.

The administration's Task Force on Worker Dislocation appointed a subcommittee to examine the foreign experience with advance-notice laws. It concluded, contrary to Mr. Sprinkel's speculation, that there is no evidence these laws have inhibited structural adjustments in Europe, and that European employers do not oppose them, but accept them as reasonable, even positive, requirements.

The real ''back-door tax'' is created by plant closings and mass layoffs that occur without adequate notice to workers and communities. Sudden closings make worker adjustment programs like the $980 million program Mr. Reagan has endorsed ineffective, if not futile. Those tax dollars will be wasted without a notice law. Even 30 days' notice, a benefit received by less than one worker in five, reduces the average worker's spell of unemployment by 25 percent, saving the taxpayer 25 percent of the cost of unemployment compensation. Annually, sudden closings increase unemployment insurance costs (and taxes) by about $300 million.

The ''tax'' imposed by sudden closings falls hardest on workers -- a fact Mr. Sprinkel ought to know but failed to mention. One study, cited by the national academies of sciences and engineering in their recent report endorsing a federal advance-notice law, found the economic loss suffered by dislocated workers to be $4,500 to $15,000 higher when they were terminated without advance notice. So much for ''hidden costs on . . . workers.''

Advance-notice laws are both humane and economically efficient. The Senate bill's flaws (which are not shared by the House bill) are in its loopholes, its narrow coverage and the shortness of its notice period, all of which compromise its potential to help workers and communities adjust in an orderly way to economic change.

WILLIAM D. FORD U.S. Representative (D-Mich.) Washington