Analytical software quality assurance (SQA) constitutes a significant part of the total development costs of a software system. Most estimates say that about 50 % of the costs can be attributed to defect-detection and removal. Hence, this is a promising area for cost-optimisation. Various defect-detection techniques can be used to improve the quality of software. Those techniques differ in many respects and it is consolidated knowledge that the best result in terms of defect-detection can be obtained by combining several diverse techniques. The main question in that context is then how to use those different techniques in a cost-optimal way. In detail, this boils down to the questions (1) which techniques to use, (2) in which order, (3) with how much effort, and (4) with which distribution over the components. The major problem stated by several authors is that the costs and

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Predictor models are an important tool in software projects for quality and cost control as well as management. There are various models available that can help the software engineer in decision-making. However, such models are often difficult to apply in practice because of the amount of data needed. Sensitivity analysis offers provides means to rank the input factors w.r.t. their importance and thereby reduce and optimise the measurement effort necessary. This paper presents an example application of global sensitivity analysis on a software reliability model used in practice. It describes the approach and the possibilities offered. 1.