Randomized A/B tests allow causal estimation without confounding but are often under-powered. This paper uses a new dataset, including over 250 randomized comparisons conducted in an online learning platform, to illustrate a method combining data from A/B tests with log data from users who were not in the experiment. Inference remains exact and unbiased without additional assumptions, regardless of the deep-learning model's quality. In this dataset, incorporating auxiliary data improves precision consistently and, in some cases, substantially.