AbstractThis paper reports the results of applying a multi‐model interactive computer program called SMERFS (statistical modelling and estimation of reliability functions for software) to a large air defence system project. SMERFS includes four fault‐count models and four time‐between‐failure models which were applied at the compilation unit and computer software configuration item level. The four fault‐count models yielded reasonable results supported by acceptable fits for nine of the ten compilation units. The four time‐between‐failure models yielded reasonable results with acceptable fits for all compilation units. A validation of the SMERFS results was attempted by truncating the latter developmental data from the input data, and by using the latter data in lieu of operational data. With the exception of one fault‐count model, no models accurately predicted the simulated o