This is a very important aspect of the study. They did not do a true randomized survey as participants were self-selecting (online surveys have this inherently -- well, mostly).
In the research, more than 6,000 U.S. working adults over the age of 18 were surveyed online by independent research firm Edelman Intelligence between July 29 — August 24, 2016. Among them, 2,049 were freelancers and 3,953 were non-freelancers. The survey defined freelancers as individuals who have engaged in supplemental, temporary, project- or contract-based work within the past 12 months.