This is just an Excerpt from a larger document, click here to view the entire document.Building an R&M Assurance Case (Reference 9)
An assurance case is an argument that assures some set of facts. The concept is used widely in the safety industry, especially in Europe where it forms the basis of a large number of safety regulations. Any case-based argument must be based on the general principles of argument. "An argument is a connected series of statements intended to establish a definite proposition". The statements used in the argument are generally statements about evidence that support the proposition (or claim) that the argument is attempting to put across. These statements can take a number of forms depending on the available evidence and the type of proposition being made. The statements in the argument take evidence and draw an inference from it and it is the pulling together of these inferences into an overall argument that support the proposition. Hence we have a simple model of "argument" as shown in Figure 1.
Figure 1. Argument Model as Used in an R&M Case (Click to Zoom)
There are actually three types of inferences or arguments: a) Deterministic arguments based upon axioms, proofs, logic, or prior research and experience, b) Probabilistic arguments based upon failure rates, static analysis, and/or assumptions about independence, and c) Qualitative arguments based on compliance with standards and industry practices.
To be satisfied R&M requirements must first be fully understood and a suitable strategy with defined Project Management tasks developed (the R&M Plan). This strategy should include identifying R&M risk areas and considering how these risks will be managed. R&M activities are then undertaken in order to generate evidence. Hence it is comparable in many ways to building a legal case.
It is essential that the activity results (i.e., the evidence) be reviewed progressively against their defined "success criteria", and also assessed in terms of meeting the ultimate target R&M measures. The strategy must be flexible in its approach to providing progressive R&M assurance, in that the results of R&M activities should be reviewed against the R&M requirements and the R&M Plan modified as necessary.
The R&M Case is a progressively expanding body of evidence. Starting with the initial statement of the requirements, the "R&M Case" also includes identified, perceived, and actual risks; strategies and an Evidence Framework referring to associated and supporting information; including R&M evidence and data from design activities; trials, etc.; through to in-service and field data as appropriate. It also records any subsequent changes. It thus becomes a top-level control document, summarized periodically through the issue of R&M Case Reports linked to the Evidence Framework. It records progress and remains with the equipment/system throughout its life. The R&M Case is used throughout the procurement chain from prime contractor down to individual subcontractors and suppliers to provide live documentation that progresses and records achievements in an evidence framework set against the target R&M measures.
The R&M Case provides an audit trail of the engineering considerations starting with the requirements and continuing through to evidence of compliance. It provides traceability of why certain activities have been undertaken and how they can be judged as successful. It is initiated at the concept stage in accordance with the R&M Plan, and is revised progressively throughout the system life cycle. Typically it is summarized in R&M Case Reports at predefined milestones.