In 2010, Rolf Spiker approached one of us with a query from a client concerning advisory material in IEC 61508 on the statistical evaluation of software. We realised that there is a dearth of practical guidance for those who wish to evaluate critical software statistically. We believe statistical evaluation of software is an increasingly important assurance technique. We commence with a brief introduction to some of the simpler statistics and then consider discursively the issues which arise during evaluation.