In the Bayesian engine, we show a posterior distribution, so these literally represent some density curve about what your posterior beliefs about uplift should be. The posterior probability of uplift being < 0% will be (roughly, due to some vis constraints) the pct of the area of this graph that is to the left of 0. So that's nice!
So why is it skewed?
• A reasonable experimenter might assume it's because we have some informative prior that skews results towards zero; but it
isn't this reason. We use non-informative priors, so we shouldn't see any weight from the prior dragging results towards zero.
• The real reason, is that we are showing you the posterior distribution of
uplift = (treatment_mean - control_mean) / control_mean
using a lognormal distribution. We approximate
uplift
using
log(treatment_mean / control_mean)
which simplifies computation of the posterior. Using this log-transformation results in the skew that you see.