An overview of relevant literature follows. The pilot edX course has been studied by Breslow, et al. (2013), and DeBoer, et al. (2014) use data from the same course to argue for the reconceptualization of educational variables. Kizilcec, et al.(2013) motivate and implement a nuanced student classification approach, and Koller, et al. (2013) suggest that certification should be considered in the context of learner intent. Emanuel, et al. (2013) show that 83% of survey responders taking University of Pennsylvania open online courses have 2- or 4-year college degrees, and Perna, et al. (2013) find persistence and achievement rates below 15%. Good reviews of press coverage and selected research findings are found at Nature and by Watters (2013).
 MITx courses reveal a similar pattern but are excluded because some MITx courses were opening and closing enrollment during the time period of interest.