Before revision:  In A/A tests, for a set of 48 outcome metrics commonly monitored in Facebook experiments the estimator has over 70  %DIF < lower variance than the simple difference-in-means estimator, and about 19\\% lower variance than the common univariate procedure which adjusts only for pre-experiment values of the outcome.abstract 
After revision:  In A/A tests, for a set of 48 outcome metrics commonly monitored in Facebook experiments the estimator has over 70  %DIF < lower variance than the simple difference-in-means estimator, and about 19\\% lower variance than the common univariate procedure which adjusts only for pre-experiment values of the outcome.abstract \% lower variance than the simple difference-in-means estimator, and about 19\% lower variance than the common univariate procedure which adjusts only for pre-experiment values of the outcome.