Research on Safety Evaluation of Social Systems from Security Viewpoints

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At the first stage of the project, the problem of optimal allocation of security sensors and/or guards is formulated as a graph/network problem, where the importance of the concept of multiple minimal cut set is stressed. The concept of multiple minimal cut set, firstly introduced by the authors in 1989, is a natural and generic extension of minimal cut set which is well known in graph theory and systems reliability and safety analysis. The algorithm to solve the formulated graph/network problem is developed and it is shown that the algorithm is very effective from the view point of computational complexity.At the second stage of the project, we consider the design and operation of alarm systems for security using a risk-based approach. Firstly, the optimal design of data items for the alarm system is proposed with considering its risk caused by detection errors; the optimal minimal combination of data items can be selected as that with minimal risk. Secondly, considering the operation of an alarm system, it is very important to maintain the reliability of alarm system. To determine the maintenance time systematically, the risk-based approach is proposed as: The maintenance should be performed if the risk caused by detection errors is larger than the desired value. This criterion always keeps the risk level reasonable and minimizes the unavailable time.