Returns to effort: experimental evidence from an online language platform
通过随机分配在线语言学习平台上的课程数量，利用工具变量法估计努力对测试成绩的因果效应，发现完成9节Duolingo课程（约60分钟）可使成绩提高0.057-0.095个标准差，表明远程学习对大学生入门语言课程的效果可与面授相当。
Abstract While distance learning has become widespread, causal estimates regarding returns to effort in technology-assisted learning environments are scarce due to high attrition rates and endogeneity of effort. In this paper, I manipulate effort by randomly assigning students different numbers of lessons in a popular online language learning platform. Using administrative data from the platform and the instrumental variables strategy, I find that completing 9 Duolingo lessons, which corresponds to approximately 60 minutes of studying, leads to a 0.057–0.095 standard deviation increase in test scores. Comparisons to the literature and back-of-the-envelope calculations suggest that distance learning can be as effective as in-person learning for college students for an introductory language course.