segmentation - 看treatment group和control group有没有区别可以分channel看 （e.g.找工作不同的渠道）
event based: - 同一个人每次看会变 better when the change is not visible to avoid disruption of experience, short term
cookie (user) based - 看过这个feature的永远保持在一组: long-term, 人对于这个feature的影响
invariant metric
