  Feedback has a powerful influence on learning, but it is also expensive to
provide. In large classes, it may even be impossible for instructors to provide
individualized feedback. Peer assessment has received attention lately as a way
of providing personalized feedback that scales to large classes. Besides these
obvious benefits, some researchers have also conjectured that students learn by
peer assessing, although no studies have ever conclusively demonstrated this
effect. By conducting a randomized controlled trial in an introductory
statistics class, we provide evidence that peer assessment causes significant
gains in student achievement. The strength of our conclusions depends
critically on the careful design of the experiment, which was made possible by
a web-based platform that we developed. Hence, our study is also a proof of
concept of the high-quality experiments that are possible with online tools.
