Which software modules have faults which will be discovered by customers?

Software is the medium for implementing increasingly sophisticated features during the maintenance phase as successive releases are developed. Softwar e quality models can predict which software modules are likely to have faul ts that will be discovered by customers, Such models are key components of a system such as Enhanced Measurement for Early Risk Assessment of Latent D efects (EMERALD), It is a sophisticated system of decision support tools us ed by software designers and managers at Nortel to assess risk and improve software quality and reliability of legacy software systems. This paper rep orts an approach to software quality modelling that is suitable for industr ial systems such as EMERALD. We conducted a case study of a large legacy telecommunications system in th e maintenance phase to predict whether each module will be considered fault -prone. The case study is distinctive in the following respects, (1) Fault- prone modules were defined in terms of faults discovered by customers, whic h represent only a small fraction of the modules in the system. (2) We deve loped models based on software product and process metrics that can make us eful predictions at the end of the coding phase and at the time of release, (3) The modelling approach is suitable for very large systems. We anticipa te that refinements of this case study's models will be incorporated into E MERALD. A similar approach could be taken for other systems. Copyright (C) 1999 John Wiley & Sons, Ltd.