This thesis bridges the gap between two popular methods of achieving more efficient online experiments, sequential tests and variance reduction with pre-experiment data. Through simulations, it is shown that there is efficiency to be gained in using control-variates sequentially along with the popular mixture Sequential Probability Ratio Test. More efficient tests lead to faster decisions and smaller sample sizes required. The technique proposed is also tested using empirical data on users from the music streaming service Spotify. An R package which includes the main tests applied in this thesis is also presented.