A new study has found that five-sixths of the population has adequate basic hospital insurance covering room and board for two weeks, but only a third of all Americans have what a panel called "full" health coverage.

The study, released yesterday, found that 85 percent of the population in 1977 was covered by private insurance or public medical programs for basic hospital room and board costs, and 89.5 percent for inhospital laboratory and X-ray fees.

But only 29.4 percent had coverage protecting them adequately against "catastrophic" or "major medical" expenses.

The study is one of the first to explore the adequacy of health benefit coverage. While the rule of thumb for the past few years is that about nine-tenths of the population has coverage of some type, the problem has been to measure how many people are covered by private insurance or government programs like Medicare and Medicaid that provide not merely some benefits but a broad range of protection at adequate levels.

The study released yesterday was done by Stephen Sudovar and Patrice Hirsch Feinstein of Pracon Inc., a local think tank, under a grant from Roche Laboratories, a subsidiary of the drug firm Hoffmann-LaRoche. It is based on figures for 1977.

Sudovar said the study shows that while most people lack adequate coverage on the full range of benefits on some of the most important types of protection, like hospital room and board and X-rays, good coverage is widespread.

This assertion was attacked, however, by Max Fine, head of the Committee for National Health Insurance, a group which supports the comprehensive national health insurance proposal sponsored by Sen. Edward M. Kennedy (D-Mass.).

The high coverage percentages for these items are "misleading" and "inflated," Fine said, because the study classifies as "adequate" some benefits which are "far too low" and are "minimal and meager." By pretending that a bad benefit is actually adequate, he said, you can make it look like many people with poor policies have adequate coverage.

The definitions of adequacy used in the study were developed by a panel that included former congressman William Roy (D-Kan.), a doctor, and Alain Enthoven, a leding health-care theoretician and former Pentagon aide. Sudovar and Feinstein concluded:

About 85.3 percent of the U.S. population was protected -- under private policies or government programs like Medicare and Medicaid -- for at least 80 percent of hospital room-and-board costs for a minimum of 15 days per spell of illness.

About 85.1 percent was covered for "some" of the costs of inpatient psychiatric care -- but there was no specified minimum in the definition of adequacy.

About 89.5 percent of the population was covered for 80 percent of the costs of inhospital laboratory and X-ray fees.

Nearly two-thirds of the population was covered by maternity care provisions.

Some 49.6 percent was covered by reimbursement for hospital visits by a doctor but this didn't include the cost of operations, physical therapy and other medical procedures -- only visits.

Only 40 percent was covered for out-of-hospital doctor costs and 44 percent for required skilled and intermediate nursing-home care.

Only 29.4 percent of the population had "catastrophic" (major medical) protection which combined three key features needed for adequacy. These are that the insurance would pay up to $250,000 if needed in the case of a long and protracted condition; would guarantee that out-of-pocket costs for the insured in the case of unusual medical expenses wouldn't exceed between 10 and 30 percent of annual income; would cover major costs, like those for surgery, chemotherapy, inpatient drugs, private nursing, which met the first two conditions and weren't covered by other policy clauses.