The material here is the start of material to support the paper on a process-based approach to safety assurance.
For a white paper on the process-based approach to safety assurance right click here for pdf (200k) or zipped rtf (37k)
For the webslides for a presentation to the Safety Critical Systems Symposium 2002, click here.
The figure below is a charicature of the present approach to safety assurance, where the high-level metric is the quantity of documentation produced (e.g. PCSR's, POSR's). The examination of other possible metrics leads to the proposals that follow.
The paper addresses the question of what information can be used through life, but particularly during design development, to provide assurance of safety. The proposition is that there is a temporal and logical dependency between the metrics shown on the manager's wallchart. Culture brings about processes which bring about product characteristics which lead to acheived performance.
It is proposed that culture metrics are best kept informal to prevent them becoming political. At a working level, culture change is likely to be the subject of initiatives rather than metrics.
Process metrics can be used in a formal context, and provide the indicator with the greatest lead.
The problem with estimated performance is that it provides little comfort early in the lifecycle, because of the inevitable uncertainties associated with early Fault Tree Analysis, as shown below. Even adding confidence limits to event probability estimates is unlikely to provide the level of assurance necessary, and may even backfire.
Localised Process Improvement (PI) or Continuous Improvement (CI) metrics related to process capability (levels 1 to 5) can be used at a team level. The designer's aide memoire to provide day to day support to safe design is likely to be a manageable set of design principles.
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