Most of this week's columns have been devoted to health insurance advice from experts in the field. The idea was to help U.S. workers and retirees pick the best health plan for 1988. The insurance open season ends Dec. 11.

If the series seemed like insurance overkill remember this is the one time per year period when most people can switch health plans. Although premiums are going up an average of 31 percent in 1988 experts say that picking the right plan can save you $1,500 next year. Conversely, picking the wrong plan could cost you $1,500 in premiums and out-of-pocket payments.

Most of the half million federal workers and retirees here can pick from 20 to 25 health plans. Choices range from nationwide fee-for-service plans such as Blue Cross-Blue Shield and Aetna to plans offered by unions and special groups and a growing number of low-cost health maintenance organizations, or HMOs.

So far we've covered what experts rated "best buys" for these groups: families, retirees, persons who anticipate heavy medical costs and persons who may leave the government next year, or whose dependents may not be eligible for the government's group health plans.

Today's column is aimed at people who have, in many ways, the most options. They are unmarried individuals, or married people who want one- person coverage. In many cases their best buy is from an HMO.

The ratings come from Checkbook's Guide to 1988 Health Plans. The book is available from the Washington Consumers Checkbook ($5.95) at 806 15th St. NW, Washington, D.C. 20005.

Checkbook's best buys are determined by looking at benefits offered, premiums and likely out-of-pocket costs to you. Today's ratings for singles are limited to plans open to all workers and retirees and do not include plans limited to certain groups of workers, which often offer excellent coverage. Checkbook's ratings include likely total 1988 costs for premiums, fees, if any, and out-of-pocket costs for average medical expenses. Checkbook suggests you look at:

CareFirst, cost to you $420 per year; MD-IPA ($440); Columbia ($490); George Washington high option ($520); Kaiser ($520); CapitalCare ($550); Health Plus high option ($620); Network high option ($670); PhysiciansCare ($760). Even the highest average cost listed by Checkbook for HMOs, such as Group Health and the HealthPlus standard options, are only $870 per year, which is less than even the least expensive fee-for-service plans.

Checkbook ratings for fee-for-service plans include Blue Cross-Blue Shield standard ($940); GEHA ($940); Postal Supervisors ($1,000); National Treasury Employees standard ($1,030); American Postal Workers and National Federation of Federal Employees standard ($1,040); American Federation of Government Employees, Alliance standard and Postmaster standard, (all $1,100) National Association of Letter Carriers ($1,120) and Aetna standard ($1,140).

The most expensive plans, according to Checkbook's ratings, are NTEU high option ($1,450); Postmaster high option ($1,460); Blue Cross-Blue Shield high option ($1,580), and Aetna high option ($1,780).Job Mart

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