Heckman, J. J.; Vytlacil, E. J. (1999). Local instrumental variables and latent variable models for identifying and bounding treatment effects. Proceedings of the National Academy of Sciences, 96(8), 4730–4734.
doi:10.1073/pnas.96.8.4730
url to share this paper:
An interview with Sci-Hub Founder Alexandra Elbakyan Who exactly should pay for academic research