In statistical testing, a model is developed to characterize the popul
ation of uses of the software, and the model is used to generate a sta
tistically correct sample of all uses of the software. A software 'usa
ge model' characterizes the population of intended uses of the softwar
e in the intended environment. Statistical testing based on a software
usage model ensures that the failures that will occur most frequently
in operational use will be found early in the testing cycle. The usag
e model is based on the software specification. The model can be devel
oped in parallel with the software, thus shortening the elapsed time r
equired to develop the deliver software. Usage modeling has been demon
strated to be an activity that improves the specification, gives an an
alytical description of the specification, quantifies the testing cost
s and, with statistical testing, provides a basis from which inference
s of software reliability may be made. This paper describes the justif
ication for statistical testing of software using a usage model, descr
ibes procedures for developing and using a usage model and discusses s
everal usage modeling issues and recent advances in usage model applic
ations.