在「An efficient bandit algorithm for real-time multivariate optimization」這邊提到了 Amazon 不是走傳統的 A/B testing，而是同時進行多變數的最佳化：
Consider the problem of trying to find a near-optimal version of a promotional message such as this one, which has 5 variable parts and 48 different combinations in total.
在這樣的測試數量下，作者預估需要 66 天才能夠得到有效的結果，而這也表示當變數更多的時候問題就更大了：
Based on the Amazon success rate and traffic size, the authors calculated it would take 66 days to conduct a 48 treatment randomized experiment. Often this isn’t practical.
也就是開頭提到的，如何一個禮拜就提昇 21% conversion rate：
Aka, “How Amazon improved conversion by 21% in a single week!”
作者也提到了這個方法其實打臉了他先前提到的另外一篇論文，在 2014 年提出測試應該要盡可能簡單 XDDD：
Yesterday we saw the hard-won wisdom on display in ‘seven myths‘ recommending that experiments be kept simple and only test one thing at a time, otherwise interpreting the results can get really complicated.
只能說狀況愈來愈複雜，導致需要新方法解決問題。而且這些電商會遇到在測試時不同的 factor 之間有可能會有相依性 (也就是說這些 factor 不是 i.i.d.)，你用本來的方式反而會測不出來。