Before revision:  Under a common experimental design that randomly assigns units into the experimental group, our method can reduce the variance by up to 50\% , so that only half of the experimental sample is required to attain the same accuracy. This accuracy can be further improved if the experimental design accounts for this relevant first-stage covariate and select the experimental group differently .
After revision:  Under a common experimental design that randomly assigns units into the experimental group, our method can reduce the variance by up to 50\% , so that only half of the experimental sample is required to attain the same statistical precision. If researchers are allowed to design experiments differently, I show that they can further improve the precision by directly leveraging this correlation between characteristics and assignment. I apply the method to a search listing dataset from Expedia that studies the causal effect of search rankings, and show that my method can substantially improve the precision .