According to the Bureau of Labor Statistics' household survey, part-time jobs jumped by 174,000 to 28,233,000 in July.
Full-time workers climbed by 92,000.
Full-time workers are down from the May jobs report while part-time workers are up for the same time period.
Workers are generally considered to be "full time" if they work over 30 hours per week.
Some economists have speculated the shift from full time to part time derives from the employer mandate in the Affordable Care Act. Under the legislation, employers will be required to offer health insurance or face penalties (though the White House will delay enforcement until 2015).
As a result, many companies have threatened to reduce full-time staff to below the 50-employee threshold for the mandate or scale back hours.
But the effect of the employer on today's part-time figures remains unclear. In fact, some economists were more optimistic when part-time workers surged last month, arguing that the ratio of part-time to full-time was seasonal and expected.
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