Immigrants have been linked to greater invention and innovation. According to one report, "immigrants have started more than half (44 of 87) of America's startup companies valued at $1 billion dollars or more and are key members of management or product development teams in over 70 percent (62 of 87) of these companies." One analysis found that immigrant-owned firms had a higher innovation rate (on most measures of innovation) than firms owned by U.S.-born entrepreneurs. Research also shows that labor migration increases human capital. Foreign doctoral students are a major source of innovation in the American economy. In the United States, immigrant workers hold a disproportionate share of jobs in science, technology, engineering, and math (STEM): "In 2013, foreign-born workers accounted for 19.2 percent of STEM workers with a bachelor's degree, 40.7 percent of those with a master's degree, and more than half—54.5 percent—of those with a PhD"