hope this fix your issue As javadoc states, it's value of

confidence interval.

Roughly speaking, assume you have two distributions (aka benchmark runs) of two methods run times. By default, JMH assumes it's normal distribution with some parameters (mean and variance). But distributions aren't numbers: you can't compare one mean with another and say "Hey, first mean is smaller than second one, thus first approach is better in average!". First you should prove that they don't belong to the same distribution (it's still possible even if they (samples, not distributions) have slightly different means), otherwise such comparison makes no sense. For such proves special

statistical test is used. But as long as test is dealing with samples, not actual distributions, test can't say "Two data sets don't belong to the same distribution", test can only say "Two data sets don't belong to the same distribution with 99% probability". This 99% (or any other) actually is confidence you are asking.