U.S. workers are increasingly working a contract or freelance “gig” instead of having traditional employer-employee relationship, according to a new survey published Monday.

The number of people working in “alternative arrangements,” that is, working as contractors, as freelancers, through a help agency, or through on-call centers, increased by nearly 60 percent from 2005 to 2015, the National Bureau of Economic Research said in a paper written by Harvard labor economist Lawrence Katz and Princeton economist and former Obama adviser Alan Krueger.