Abstract: Methods, systems, and apparatus, including computer programs encoded on computer-storage media, for imbalance detection in online experiments. In some implementations, a method includes obtaining user information indicating a first set of devices assigned a first version of a service and a second set of devices assigned a second version of the service in a multivariate testing framework; generating alternative samplings of devices; generating a threshold for detecting imbalance using the alternative samplings of devices, a generated expected first set of devices, and a generated expected second set of devices; detecting an imbalance using the obtained user information indicating the first set of devices, the second set of devices, and the generated threshold; and implementing a corrective action to mitigate the detected imbalance in the multivariate testing framework.