Answers are grounded in this paper and cite their sources.
Joshua D. Angrist, Guido W. Imbens, Comment on James J. Heckman, "Instrumental Variables: A Study of Implicit Behavioral Assumptions Used in Making Program Evaluations", The Journal of Human Resources, Vol. 34, No. 4 (Autumn, 1999), pp. 823-827
No takes yet. Share an insight, caveat, or question.
Angrist et al. (1999) studied this question.
Synapse has enriched 10 related papers. Consider them for comparative context: