One of the best things about using Stata is its excellent documentation. Since version 11 Stata has come with comprehensive PDF documentation (prior to that you had to buy the manuals separately, which many chose not to do). Read -help roccomp- and then click on the link to the PDF documentation. As an example, for my Stata 12 installation the -roccomp- manual entry provides 12 pages of documentation including many examples, all of the formulae used and a bunch of references.