A model for optimal information retrieval over a distributed document collection is described and experimentally evaluated. The fusion of retrieval results corresponding to document subcollections is performed according to the Probability Ranking Principle. Part of the model is a selection criterion for e ectively limiting the ranking process to a subset of subcollections.

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Background: Different events affecting public transportation systems such as the Madrid bomb attacks in 2004 have exposed the vulnerability of open and complex infrastructure. Providers have to deal with these threats and prepare to prevent and respond. For a systematic identification and assessment of threats, a variety of appropriate risk management (RM) systems can be chosen. To choose an appropriate RM system for a public transportation system, a comparison is needed. Therefore, the providers have to define self-assessment criteria based on local requirements to get reliable results.Aim: Most international RM systems are based on qualitative descriptions making it difficult to extract and determine such criteria. In fact, no comparison can currently be done between different qualitatively described RM systems. One solution to obtain reliable comparative results is the quantification of qualitative statements. This research shows an assignment of numerical values for qualitative criteria to make a comparison possible.Method: The comparison is based on adaptation of a use-value analysis that accomplishes the requirements of public transportation systems. It compares different decision alternatives, especially if there are no quantitative criteria. Qualitative criteria are categorized and assigned to each pre-defined target compliance factors connecting with numerical values. Additionally, the prioritisation of qualitative criteria is determined with weighting coefficients. Results: The combination of weighting coefficient and target compliance factor helps to establish reliable results in a relative comparison. The use-value analysis is defined by assessment criteria for risk and threat detection, effort of implementation and support, and quality of RM results.Conclusion: Public transportation providers can use this method using their local requirement and assessment criteria to find an appropriate RM system. Still, the application of the use-value analysis provides reliable information about the suitability of different RM systems, but no further information about the effect of the systems.

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