Before revision:  Our proposed technique is a practical alternative to standard methods and allows the researcher to make meaningful inferences about the structural function when some misspecification cannot ruled out. Our procedure is easy to implement and computationally light. The first stage resembles that of sieve minimum-distance estimation, and the second stage is formulated as a linear program .
After revision:  Our proposed technique is a practical alternative to standard methods and allows the researcher to make meaningful inferences about the structural function that are valid under a small failure of instrumental validity. We construct a simple, consistent estimator of the identified set .