Software testing is an important and a costly phase of the software development iifecycle. In this paper, an approach is presented that improves the eflectiveness of software testing based on partitioning the input domain. Specijcally, it is shown how partitioning can decrease the number of tests needed to find defects. The improvement is based on partitioning the input domain according to how different user-level functions of sofnyare read from and write to internal state variables. Several

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... ameters that influence the probability of finding defects are investigated with a computer simulation: the size of a test case, the type of a defect, the number of partitions, and the ratio of reading and writing accesses to state variables.