Istituto di Scienza e Tecnologie dell'Informazione     
Bertolino A., Strigini L. On the use of testability measures for dependabily assessment. Report SHIP/IEI/004. Internal note IEI-B4-44, 1994.
Program "testability" is, informally, the probability that a program will fail under test, ifit contains at least one fault. When a dependability assessment has to be derived from the observation of a series of failure-free test executions (a common need for software subject to "ultra-high reliability" requirements), measures of testability can be used to draw inferences on program correctness (and hence on its probability of failure in operation). In this paper, we rigorously investigate the concept of testability and its use in dependability assessment, criticising and improving on previously published results. We first give a general descriptive model of program execution and testing, on which the different measures of interest can be defined. We propose a more precise definition of program testability than that given by other authors, and discuss ways to incre se testing effectiveness without impairing program reliability in operation. We then study the mathematics of using testability to estimate, from test results: i) the probability of program correctness and ii) the probability of failures. To derive the probability of program correctness, we use a Bayesian inference procedure and criticise previously proposed derivations of this probability. By studying the relationship between the testability of a program and its failure probability in operation, we derive the conditions under which a higher testability can effectively improve one's expectations about program reliability. The potential of these results for practical application is discussed throughout the paper.
Subject dependability assessment
D.2.5 Testing and Debugging

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