The help-wanted ads in Sunday newspapers must be one of President Reagan's favorite economic indicators. He returns to them time and again to make a favorite point: that there are lots of good jobs available and going begging.

One possible conclusion is that some of the jobs are available because some people don't want to work.

That's a theory the president subscribes to, as he pointed out in his press conference Tuesday, when he recalled a program he ran as governor of California a decade ago, which ordered able-bodied welfare recipients to show up for work on "community chores."

And that's one way to tackle the problem of long-term unemployment among welfare recipients, and undereducated, undertrained poor people.

But there's another reason why large numbers of good jobs remain unfilled -- a mismatch involving people who lack the job skills that employers are looking for.

It's a problem about which the president isn't particularly well-informed, apparently, based on his misunderstanding of a Massachusetts job-training program that he was asked about at the press conference. But it is a problem, even after three years of economic recovery.

One measure of that problem is the numbers of displaced workers who have lost their jobs because of plant closings or relocations, automation and declining industrial output.

A new congressional study reports that, between 1979 and 1984, 11.5 million American workers lost jobs for these reasons.

Nearly half -- 5 million -- had held their jobs for at least three years, a work record that makes them "displaced workers," in the government's jargon.

Of these displaced workers, 1.3 million were unemployed in January 1984 and 500,000 of them had been unemployed for more than six months.

During the five-year period, nearly one-fourth of the displaced workers were without jobs for more than a year, according to special survey in 1984 by the Census Bureau, analyzed in a new report on unemployed issued by the Office of Technology Assessment.

These displaced workers have been the primary targets of the federal government's employment aid, a fact the president pointed out. The program he referred to is the 1982 Job Training Partnership Act, which the OTA report called "the first comprehensive program for displaced workers in nearly 20 years." Unlike the programs it replaced, it gave control to state governments and placed responsibility for designing the training on local business.

The OTA report concluded that it's still too early to assess the effectiveness of the Job Training Partnership program.

In its first year, 72 percent of the participants said they "graduated" to new jobs, and 65 percent did the same in the second year. The figures are comparable for government training programs in the 1960s, OTA said.

But assuming there were about 3 million displaced workers unemployed in 1983, its first year, the program reached only 4 percent of those eligible.

Why the participation wasn't higher isn't clear, the congressional study said. Unemployed workers may not have known about the program, or didn't think they needed the help, or doubted the program would do any good.

If retraining is the issue, the program doesn't offer much. Instead, it emphasizes getting jobless people into new work quickly. There's a budgetary benefit from that: It costs several thousands of dollars to train people for many kinds of entry-level jobs, and the per-person cost of the Job Partnership Training program is well below that level.

Despite the president's tribute to the program, it hasn't been immune from budget cutting. Two years after the program began, Congress voted to cut funding by 55 percent for the current fiscal year, OTA reported.

And as important as JPTA is for displaced workers who do enroll, this primary government jobs program misses a large part of the population who need the help.

A profile of enrollees in 1984 shows that 62 percent were male, 30 percent minority, 94 percent were 22 years of age or older and 80 percent were high-school graduates.

The program skips over the large and growing numbers of people with little or no connection to the job market -- mothers of dependent children whose support comes from welfare payments.

At the press conference, the president was asked about another federal training program, this one intended to help welfare recipients enter the job market. Massachusetts Gov. Michael Dukakis noted recently that the program has found work for 23,000 former welfare recipients since October 1983, after providing them with job training and day care.

The jobs are not make-work, said Charles Atkins, commissioner of the Massachusetts Department of Public Welfare. Two-thirds of the 23,000 work full time; the rest work part time as assemblers in high-tech manufacturing and as clerks, and earned an average of $10,000 a year.

"The private sector is gobbling them up," said Atkins, whose state's unemployment rate is only 3 percent.

After roughly one year of the program, more than 85 percent have not returned to welfare, Atkins' department says.

Finally, it is a bargain for the federal government: For each $1 of federal aid committed to the program, it has returned $2 in reduced welfare costs, Massachusetts officials say, a ratio that holds true in the rest of the country as well.

Those benefits, however, haven't spared the program. The president's budget for fiscal 1987 cuts off funding for this program. At the press conference, the president did not explain why.