fers three key approaches (Gilb, 1986) to achieving quality under deadline pressure: Albrecht's metric of function points (Albrecht, 1983) allocates scores to known sources of complexity, such as inputs and outputs. 1. Evolutionary delivery, not attempting to provide everything This offers a measure of size from which cost estimates can be asked for at once. inferred. The size metric corresponds rather more to the scale of the 2. Inspection, especially of the early specification and design problem rather than the way it is programmed. outputs (Fagan, 1976). 3. Setting measurable quality objectives. 25.11 Software quality and reliability 25.12 References 25.11.1 Measuring quality Albrecht, A.J. and Gaffney, J.E. (1983) Software function, source One of the difficulties in trying to achieve quality in a product is that lines of code and development effort prediction: a software of knowing what constitutes quality. To see whether quality is science validation, IEEE Transactions on Software Engineer- sufficient, and to check that there is a trend of improving quality, ing, SE-9, (6), November. there need to be measures. However, quality is not single valued; Araki, A., Furnjawa, Z. and Cheng, J. (1991) A general framework there have to be a number of quality factors, which may be interde- for debugging, IEEE Software, 8, (3), May. pendent but which are distinct. Bal, H.E., Steiner, J.G. and Tanenbaum, A.S. (1989) Programming The following set of quality factors is due to McCall, Richards and languages for distributed computing systems, ACM Computing Walters (McCall): correctness, reliability, efficiency, integrity, usa- Surveys, 21, (3), September. bility, maintainability, testability, flexibility, portability, reusability, Bersoff, E.H., Henderson, U.D. and Siegel, S.G., (1979) Software interoperability. configuration management: a tutorial, Computer, January. However, these still do not lend themselves to easy measurement. Birtwistle, E.M. (1985) Discrete event modelling on Simula, Mac- One proposal for obtaining me