About 34 million Americans, nearly one-sixth of the population, lack full-time year-round health insurance coverage, although some have it for part of the year, according to a study by the Department of Health and Human Services.

The survey, conducted by HHS's National Center for Health Services Research and based on a sample of about 40,000 people in 1977, is probably the most comprehensive ever done of health insurance coverage for the U.S. population.

It shows that blacks, Hispanics, poor people, young adults, farmers and residents of the West and the Sun Belt had the worst coverage rates.

The major findings:

177.8 million people, or 83.9 percent of the civilian noninstitutional population, had full-time year-round coverage.

34.2 million lacked such coverage, but only 18.2 million were completely uncovered all year. The other 16 million, such as people employed only part of the year, had coverage during some parts of the year but not the whole year. The study estimated that taking into account both these groups, at any given time of the year about 25 million people were uncovered.

Taking the population by age group, the study shows that young adults 19 to 24 had the lowest fulltime, year-round coverage rates: 70 percent. This is in sharp contrast to people 65 and over, who are generally protected by Medicare and whose year-round coverage rate was 97.8 percent.

The study showed little difference in coverage by sex: both males and females had essentially the same year-round coverage rates, about 84 percent.

By race there were major differences: 86 percent of whites had year-round coverage, but only 77 percent of blacks, 76 percent of Hispanics and 72 percent of other minorities.

Nine-tenths of all high-income people had year-round coverage, but the figure dropped as income dropped, and was only 73 percent for the poor.

Among occupations, white-collar coverage was highest, 87 percent, and farmer coverage lowest, 78 percent.

And while close to nine-tenths of of the people in the North had year-round coverage, the figure was only 79 percent for the South and West.

The study did not evaluate how good any of the health insurance plans were in benefits, scope of protection and the like, but only whether people had some form.

However, studies by other groups have shown that the range of protection varies enormously from policy to policy: some cover only bare- bones hospital room-and-board charges and in-hospital surgical costs, while leaving out hospital "ancillary" charges (lab fees, radiology, etc.) that average as much as the room-and-board fee, out of-hospital doctor fees, medicines and the like.

Others, by contrast, are quite generous, including the ancillary charges, out-of-hospital doctor and drug costs, dental charges, psychiatric fees and even some form of cap on total annual out-of-pocket charges to the patient in event of catastrophic illness.