Comment on James J. Heckman, "Instrumental variables: A study of implicit behavioral assumptions used in making program evaluations''

In a recent paper in this journal, Heckman discussed the use of instrumental variables methods in evaluation research and our local average treatment effects (LATE) interpretation of instrumental variables estimates. This comment provides additional background for Heckman's paper, and a review of our rationale for focusing on LATE. We also show that a set of assumptions proposed by Heckman as an alternative to the LATE assumptions are not compatible with either latent-index assignment models or the definition we proposed for an instrument.