https://www.lesswrong.com/posts/9kNxhKWvixtKW5anS/you-are-not-measuring-what-you-think-you-are-measuring

Eight years ago, I worked as a data scientist at a startup, and we wanted to optimize our sign-up flow. We A/B tested lots of different changes, and occasionally found something which would boost (or reduce) click-through rates by 10% or so.

Then one week I was puzzling over a discrepancy in the variance of our daily signups. Eventually I scraped some data from the log files, and found that during traffic spikes, our server latency shot up to multiple seconds. The effect on signups during these spikes was massive: even just 300 ms was enough that click-through dropped by 30%, and when latency went up to seconds the click-through rates dropped by over 80%. And this happened multiple times per day. Latency was far and away the most important factor which determined our click-through rates. [1]

Going back through some of our earlier experiments, it was clear in hindsight that some of our biggest effect-sizes actually came from changing latency - for instance, if we changed the order of two screens, then there’d be an extra screen before the user hit the one with high latency, so the latency would be better hidden. Our original interpretations of those experiments - e.g. that the user cared more about the content of one screen than another - were totally wrong. It was also clear in hindsight that our statistics on all the earlier experiments were bunk - we’d assumed that every user’s click-through was statistically independent, when in fact they were highly correlated, so many of the results which we thought were significant were in fact basically noise.