Millions of Americans don’t want to work part-time.

The U.S. economy added just 98,000 jobs in March, the smallest gain in nearly a year, after adding more than 200,000 jobs in January and February. Economists predicted that the number of jobs created in March would hit 180,000, so the actual figures fell far short of that. Unemployment fell to a 10-year-low of 4.5% in March from 4.7% in February, but the “real” unemployment rate that includes part-time workers who would rather work full-time and job hunters who gave up searching for work was 8.9%, although this was also down from 9.2% in February.

Part-time work is still a contentious alternative for many workers. On Thursday, Amazon said it will create 30,000 part-time jobs in the U.S. over the next year, nearly double the current number. Of those, 25,000 will be in warehouses and 5,000 will be home-based customer service positions. Amazon AMZN, -3.18% said in January it would create 100,000 full-time jobs over the next 18 months, according to a separate announcement made in January. Last year, Amazon’s world-wide workforce grew by 48% to 341,400 employees. In the U.S., it has over 70 “fulfillment centers” and 90,000 full-time employees. (Amazon did not respond to request for comment.)

There were some 5.6 million involuntary part-time workers in March 2017, little changed from the month before, but down from 6.4 million a year earlier, according to the Bureau of Labor Statistics. That number is up from 4.5 million in November 2007, but way off a peak of 8.6 million in September 2012. These figures are almost entirely due to the inability of workers to find full-time jobs, leaving many workers to take or keep lower-paying jobs, according to the Economic Policy Institute, a nonprofit think-tank in Washington, D.C. And 54% of the growth in these involuntary part-time jobs between 2007 and 2015 were in retail, leisure and hospitality industries, the EPI said.

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There’s a prolonged “structural shift toward more intensive use of part-time employment,” the Economic Policy Institute report found. Aside from the frequent lack of sufficient work hours, these part-time workers must also “navigate unpredictable and/or variable hours,” with their work schedules varying week-to-week at a rate more than double that of full-time workers, it added. What’s more, part-time workers suffer from a lower rate of pay and benefits coverage than full-time workers, such as access to health insurance and paid time off. Compared to similar full-time workers, men working part-time earn 19% and women working part-time earn 9% per hour.

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There’s no clear association with the implementation of the “employer mandate” in Obamacare and the time trend in involuntary part-time workers, said Lonnie Golden, economics professor at Penn State University and author of that EPI report. (The employer mandate requires certain businesses to provide health insurance when their employees work a set amount of hours.) Golden said the data does not reflect any noticeable increase in involuntary part-time or those working 29 or fewer hours a week. While 3.7% of white Americans work part time involuntarily, 6.8% of Hispanic workers and 6.3% of black people work part-time, but would rather be full-time, the report concluded.

Perhaps not surprisingly, involuntary part-time workers tend to earn less than their voluntary part-time counterparts. Approximately 40% of involuntary part-time workers report a total family income of less than $30,000, compared with just 18% of the latter and 29% of the population as a whole, according to an earlier report — “A Tale of Two Workforces: The Benefits and Burdens of Working Part Time” — published in 2015 by Rutgers University. More than four out of every five involuntary part-time workers says it’s hard to save for retirement and about seven out of every 10 say they earn less money than they and their family need to get by and pay bills.

The persistence of such large numbers of involuntary part-time workers is an indicator of underlying weaknesses in the U.S. labor market, experts say. Involuntary part-time workers are twice as likely as voluntary part-time workers to be forced to work on weekends and holidays, and to be given unfavorable work schedules and job assignments, the Rutgers report concluded. In 2015, then-Presiden Barack Obama introduced measures to require U.S. companies to pay overtime to millions of salaried workers, but that move was halted by a federal judge last December. Some companies introduced overtime for those workers, regardless.