Modeling Software Behavior in Terms of a Formal Life Cycle Curve: Implications for Software Maintenance

In this paper, a formal model of the software manloading pattern, the Rayleigh model, is described and then applied to four Bankers Trust Company (BTCo.) new development projects possessing complete life cycle manloading data (maintenance phase included). To fit the Rayleigh curve to a project's manloading scores, (nonlinear) regression was used to obtain least squares estimates of the Rayleigh parameters, which, in turn, were used to generate the Rayleigh manloading curve. For all four projects, deviation from the Rayleigh curve was small and constant throughout the software development phases (i.e., preliminary design through implementation); however, the Rayleigh curve consistently deviated from the actual manloading during system maintenance, underestimating the amount of maneffort expended. Restricting maintenance maneffort to manpower expended on repair of system faults (``corrective'' maintenance) resulted in a single Rayleigh curve that could be applied over the entire BTCo. life cycle. Furthermore, this corrective portion of the maintenance effort could be accurately forecasted from the Rayleigh curve fit to software development. Implications of these findings for software management are discussed.