PHILADELPHIA – Waiting for a job interview in the Accu Staffing office in Cherry Hill, Pa., the other day, 35-year-old Seth Rosen was seeking to catch one of the biggest waves of this economic expansion – a part-time job.

Rosen is a computer systems administrator who has been without full-time work in his profession since 2001. To earn money this summer, he resorted to digging ditches and other manual labor for a construction company.

He has plenty of company. Even as the unemployment rate has declined in 2004 and economic output is expanding, the growth in the U.S. labor market is coming from part timers – workers who clock less than 35 hours a week and typically aren’t offered health benefits.

That leads some economists to question the staying power of the recovery. “It’s premature to say the economy has recovered,” said Andrew Reamer, economist and owner of Andrew Reamer & Associates in Newton, Mass. He said that while indicators such as corporate profits and gross domestic product “look great,” there are fewer full-time workers than three years ago and 12 percent more part-timers.

Rosen has been encouraged by newspaper advertisements in recent weeks saying jobs are coming open in his field. But the jobs, at least based on his reading of fine print in those ads, seem mostly temporary or part-time that could lead to full time.

“I pick up computer jobs here and there but it is not enough to live on,” said Rosen, who is single and lives in Lindenwold, Pa.

Using July as a base month, the economy has shed about 100,000 full-time jobs from the depths of the recession in July 2001 through last month, according to the Labor Department household survey data that is not adjusted for season fluctuations. The nation has 114.3 million full-time workers.

At the same time, the economy has added 2.5 million part-timers, an 11.6 percent gain, to 24.4 million workers in the three years. This includes self-employed persons who don’t consider their jobs full time.

That means that for every eight part timers in 2001 there was an additional part-time worker earning a paycheck this year. This is a dramatic change from the mid to late-1990s, when part-time employment remained relatively stable at about 22 million workers.

In July, the nation’s unemployment rate fell 0.1 percentage point to 5.5 percent, mostly because of a surge in part-time hiring. About 95 percent of the gains in July were part-time positions, or 577,000 of 605,000 new jobs.

Economists say several forces are behind the trend. “Businesses are getting better at figuring out how to structure work so it is most beneficial to them,” said Reamer. That means, for example, that a company concerned about the soaring cost of health-care benefits and also uncertain about the economy might offer overtime to an existing employee and hire a part-time employee to get additional work accomplished. This allows the company to avoid the extra health-care benefit costs of full-time workers.

Mark Zandi, chief economist with Economy.com in West Chester, said some growth in part-time workers reflects lifestyle choices of an aging workforce: some people do not want to put in 40 hours a week. But he said the main story is business confidence. Companies are not willing to make the commitment to add full-time employees.

Zandi and others say rising oil prices and the cost of employee benefits, in particular health-care coverage, are making companies think hard about adding full-time staff.

While the official unemployment rate is 5.5 percent, the “underemployment” rate in the nation is 9.6 percent, Zandi said. The underemployment rate calculates those people in part-time jobs but want full-time positions and those who have stopped looking for jobs because they don’t believe they would get hired. The rate has declined from 10.3 percent in mid-2003, Zandi said.

The labor market remains weaker than what economists expected this far into the recovery for other reasons. A separate payroll survey of businesses by the Labor Department shows that job growth is primarily taking place in occupations that don’t pay well, such as restaurants and retail stores, Zandi said.