[jules]

In an A/B test where you usually get so much data, priors honestly don't matter much. Just use a flat prior. You'll overestimate the uncertainty a bit, so you may need a couple more data points than necessary but it's still way less than you'd need for a frequentist method. An A/B testing company could even automatically come up with better priors based on A/B tests that their customers have done in the past.