Recent work has shown how to add covariates, under what conditions doing so is valid, and the potential for increased precision.<ref>{{cite arXiv|last1=Calonico|last2=Cattaneo|last3=Farrell|last4=Titiunik|title=Regression Discontinuity Designs Using Covariates|date=2018|eprint=1809.03904|class=econ.EM}}</ref> | Recent work has shown how to add covariates, under what conditions doing so is valid, and the potential for increased precision.<ref>{{cite arXiv|last1=Calonico|last2=Cattaneo|last3=Farrell|last4=Titiunik|title=Regression Discontinuity Designs Using Covariates|date=2018|eprint=1809.03904|class=econ.EM}}</ref> |