We propose a hybrid approach for functional safety assessments of software. By hybrid, we mean that it is both goal-based and prescriptive. Recent studies advocate goal-based assessments based on structured, rigorous safety cases. The problem is that products also need to be certified against existing, prescriptive standards. This study suggests a way to integrate the prescriptive standard with a goal-based safety case approach. The main idea is to integrate the prescriptive elements in the standard into a goal-based safety case in order to improve the transparency and consistency of the safety certification. We also propose to categorize the safety cases into safety case patterns to improve reuse of safety certifications. We exemplify our approach using the IEC61508 standard where we have transformed all the prescriptive elements in part 3 of the standard into a collection of safety case patterns.
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