Before revision:  Even when measurements of (Y,D) are confounded , the treatment effect on the subpopulation of compliers can nonetheless be identified if an instrumental variable Z is available , which is independent of (Y,D) conditional on X and the unmeasured confounder .
After revision:  Even when measurements of (Y,D) , the treatment effect on the subpopulation of compliers can nonetheless be identified if an instrumental variable Z is available , which is independent of (Y,D) conditional on X and the unmeasured confounder .