Hi All, We are using a regression discontinuity design to estimate effect of the passage of a school bond to reduce class size on actual class sizes. This gives us plausibly exogenous variation in class size. We are wondering how we might be able to use these predicted values (the exogenous part of class size) to then estimate the effects of class size reduction on student achievement. I have never seen a two stage least squares where your instrument in the first stage comes from a regression discontinuity. I know if we simply use the predicted values from the first stage and plug them into the second stage as our class size variable then we have to adjust the standard errors of the second stage equation but I don't know how to do that. It is also unclear to me if we would need to include the same control terms from the RD design in the second stage where we regress student achievement on class size. Any insights would be greatly appreciated. matt -- Matthew Kraft Doctoral Candidate Quantitative Policy Analysis Harvard Graduate School of Education