Many federal workers are being badly advised, or deliberately misled, by agencies when they ask how to get more time to select their 1980 health insurance plan.

Although the insurance open season technically ends Dec. 7 there is a way to get another month -- until Jan. 7, 1980. The key is that you must say you have been confused about which plan is best for you based on incomplete data put out by the government.

The Office of Personnel Management has told agencies to allow workers extra time if they say they were confused by "purportedly incomplete data" in a health insurance brochure put out by Health, Education and Welfare. It was distributed to workers in three cities -- Washington, Chicago and Philadelphia.

The American Federation of Government Employees sued HEW and the Office of Personnel Management on grounds that the brochure amounted to free advertising for a select group of plans. It compared benefits (but not price) offered by Aetna and Blue Cross-Blue Shield (the two largest federal health plans) with those of local Health Maintenance Organizations. It excluded information about the remaining 100 health insurance plans offered to federal and postal workers.

OPM officials -- who oversee the federal health program -- convinced U.S. District Court here that they had taken steps to inform employes that the HEW data was incomplete.But OPM also agreed to allow any employe alleging confusion because of the HEW brochure to get extra time -- up to Jan. 7 -- to make a choice.

On Nov. 27, this column reported that "confused" employes would be allowed an extra month to pick an insurance plan. It said formal notice would be coming soon from OPM. The very next day, Nov. 28, OPM sent a letter (Bulletin No. 127) to agency health insurance officers. It confirmed that the limited extension would be granted, and directed officers to explain that to workers who asked.

Despite its letter, however, OPM officials were obviously unhappy over the prospect of thousands of federal workers taking an extra month to pick insurance, or changing selections they had already made. With that in mind the column advised:

". . . You may have to be persistent to get results.Most agencies will not acknowledge the extension until OPM spells out the gospel . . . OPM is not anxious to encourage everybody to ask for the extension, or change plans if they have already picked one . . . but you do have the right to make the change if you go about it correctly."

Since then, dozens of federal workers have called this office. They have complained that their personnel, or insurance, offices said they knew nothing about an extension of the decision-time, beyond what they read in the paper. Several callers said that OPM aides told then this column "got it wrong" and told them they had to decide on insurance by the Dec. 7 deadline. Maybe the people at OPM ought to read their own bulletins. Here is the Nov. 28 letter to agencies, signed by Gary R. Nelson. He is associate director for compensation. It reads:

"The OPM has also informed the U.S. District Court that if any federal employe states that he or she enrolled, changed enrollment, or continued an enrollment in a particular health benefits plan on the bais of purportedly incomplete information contained in the HEW brochure, that employe will be permitted to change his or her enrollment into another health benefits plan.

"Employes in the three listed areas [Washington, Philadelphia and Chicago] who wish to avail themselves of this opportunity must file Standard Form 2809 with their employing office before or on Jan. 7, 1980. This bulletin does not permit any other types of changes to be made after the close of open season on Dec. 7, 1979. Any change made pursuant to this bulletin will be effective on the same date as other changes made during open season.

"We therefore direct your agency to permit changes in enrollment between benefit plans, as described above, which are made before or on Jan. 7, 1980. Please so inform employing offices at all installations which are effected by this bulletin."