The A() function calculates the A statistic, a nonparametric
measure of effect size for two independent groups that’s also known as
the probability of superiority (Ruscio, 2008), along with its standard
error and a confidence interval constructed using bootstrap methods
(Ruscio & Mullen, 2012). Optional arguments can be specified to
calculate variants of the A statistic developed for other research
designs (e.g., related samples, more than two independent groups or
related samples; Ruscio & Gera, 2013).