Shanee Greenidge of Boston has been searching for full-time work since she dropped out of high school in 2009 and took a string of part-time jobs to help her mother pay bills.

I'm looking for any type of full-time job. I don't care what it is, I really need something, said Greenidge, 20.

Her situation is typical of millions of young Americans caught up in the aftermath of the country's deepest economic crisis since the Great Depression.

Greenidge has held a number of part-time jobs in the past two years, including work as a landscaper, but nothing to put her on a permanent career path.

Even for part-time retail jobs, she said, she is competing with people with college degrees or years of experience.

There's a lot of competition. It sometimes feels like I don't stand a chance, she said.

The number of Americans working part-time because they cannot find a full-time job or because their hours were cut more than doubled from around 4 million in 2007 to more than 9 million in 2009.

The number is edging lower, but as of January 2011, 8.4 million were still working part-time because of the weak economy, according to U.S. payrolls data issued on Friday.

The U.S. unemployment rate has fallen to 9 percent, but if involuntary part-time workers and people who are not actively looking for work are counted, it stands at 16.1 percent, according to government data.

Andrew Sum, an economics professor at Northeastern University in Boston, said past recessions suggest it will take several years to make a significant dent in the number of underemployed Americans.

It takes really strong three or four years of growth until you get a big push down in this number, he said. There are a large number of employers who are not sure about future demand. So they want to keep the cost down.

But the cost of being underemployed is huge, both for those desperate for more work hours -- who tend to be young adults, less-educated and blue-collar workers -- and the broader economy, Sum said.

Most part-time employees work half the hours of full-time employees and often do not have benefits such as health insurance and pensions, Sum said. That puts a strain on already stretched public services.

Underemployed workers tend to get less training at work and earn less in the future than full-time colleagues, he said.

These lower earnings hold back their spending on goods and services, which drives the U.S. economy. Part-time workers on low incomes are also more likely to need social services such as food stamps, even as their lower wages and expenditures reduce their tax contributions, adding to U.S. fiscal strains.

Neil Sullivan, executive director at the Boston Private Industry Council, said the difficulty young people have getting a firm foothold in the job market is especially worrying.

Disconnected youth are the ones that do the most harm to themselves and the community, Sullivan said. You can find them on the street corners all around urban America and there are few prospects for them apart from part-time retail.

NO EXPERIENCE, NO JOB

Melissa Rodrigues, 25, who recently graduated with a bachelor degree in sports sciences, works part-time looking after children at an after-school club while she looks for a permanent job. Many peers who graduated with her, she said, are waitressing or going back to school.

I've applied to a lot of places, but they want experience. They want two years, for everything, she said.

Even those with more experience can find it tough to regain their footing in the labor market.

Beth Tarbell, 46, was laid off from her job writing procedures and safety manuals for the restaurant industry in Austin, Texas, last spring and since then has been working part-time, off and on.

I'm getting a bit nervous now, she said. I know people who are sleeping on other people's couches and I am hoping I don't become one of them, she said.

I've had a former boss tell me, 'I wish we could afford to bring you on full-time but we can't.'

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