We present a comment on "Benefits of completing homework for students with different aptitudes in an introductory electricity and magnetism course", by F. J. Kontur, K. de La Harpe, and N. B. Terry PRST-PER 11, 010105 (2015). Our data show that the conclusions Kontur and coworkers draw from their data may not be generally applicable.

Following an identification strategy that allows us to largely eliminate unobserved student and teacher traits, we examine the effect of homework on math, science, English and history test scores for eighth grade students in the United States. Noting that failure to control for these effects yields selection biases on the estimated effect of…