The Health and Human Services Department is devoting more study to nursing home care and community services for elderly people, and a new report from the Population Reference Bureau tells why.

Today, the report says, about one aged person in four needs nursing home care or special help at home.

There are 1.4 million elderly people in nursing homes and 5.2 million others who, while remaining at home, require help in the tasks of daily living.

About four-fifths of that help comes from friends and relatives, mainly wives and daughters.

But by the year 2020, the number of people needing nursing home care and home-health assistance will approximately double, as the population ages and the number of people over 85 increases dramatically.

To compound the problem, more women will be working and thus will be unavailable to take care of their parents at home, and there will be a lower ratio of younger people to older people -- a product of the "baby bust" that followed the post-World War II "baby boom."

What to do about it? The study's author, Alice Day, says large numbers of elderly people could be kept at home at less cost if family members were given some relief from the burden of taking care of them -- such as government aid for home help, respite care, overnight care and temporary care.

In addition, she said, policy makers should tighten screening procedures to make sure the bulk of aid goes to those who need it most.

They also should place more emphasis on preventive health care such as diet and exercise to curtail disabling conditions, institute a system of copayments to make the elderly share the costs of their care, and develop new kinds of "intermediate housing" where elderly persons could receive affordable health services.

HOSPITAL STAYS LEVELING OFF? . . . The recent downward trend in the length of hospital stays under Medicare may have leveled off.

Latest reports show that the average stay for patients under the Medicare prospective payment system was 7.7 days for the nine months ending June 1985, compared to 7.6 days for the preceding year.

That is about two days less than a few years ago, before the Reagan administration instituted its system under which hospitals know in advance how much they will receive for a patient's care.

Meanwhile, the medical care component of the Consumer Price Index increased 6.6 percent from September 1984 to September 1985, according to the Bureau of Labor Statistics.

The overall CPI increased 3.2 percent in the same period.

COMINGS AND GOINGS . . . Robert B. Wilkins Jr. has been named the new associate commissioner of the Social Security Administration for governmental affairs. He previously served as confidential assistant to the secretary of Agriculture for intergovernmental and public affairs, as well as at the Justice Department. . . . The National Association of Medical Equipment Suppliers has hired John Bailey, a former aide to former Rep. Keith G. Sebelius (R-Kan.) and the House Agriculture Committee, as its director of government relations . . . . Paul Willging, a former official of the Health Care Financing Administration, is the new executive vice president of the American Health Care Association, an umbrella group for 50 state nursing home associations . . . . Thomas R. Rollins, an attorney with the Houston law firm of Susman, Godfrey & McGowan, has been appointed Democratic chief counsel at the Senate Labor and Human Resources Committee.

RISING COSTS OF BENEFIT PROGRAMS . . . The General Accounting Office estimates that 16 major programs, in which benefits are based on need, will grow from $65.3 billion in fiscal 1983 to $70 billion in fiscal 1986 in constant, after-inflation dollars.

The calculation shows that most programs -- food stamps, welfare, rural housing, pensions for needy veterans -- will shrink somewhat in constant dollars, but that the Medicaid program, with a whopping $6.5 billion increase, will offset all the others.

The same report estimates that 26 benefit programs based on the insurance principle will rise in constant after-inflation dollars from $313.7 billion in fiscal 1983 to $318.4 billion in 1986.

Most programs will remain static in after-inflation dollars, but Social Security and Medicare, which together will total $242 billion in fiscal 1986, will each grow by $10 billion.

Unemployment insurance is projected to decline from $29.1 billion to $12.7 billion in after- inflation dollars over the same peri- od.

Some of the program changes are the result of cuts, but some result from changes in population and economic conditions.