Archives For Safety cases

Current practices in formal safety argument notation such as Goal Structuring Notation (GSN) or Cause Argument Evidence (CAE) rely on the practical argument model developed by the philosopher Toulmin (1958). Toulmin focused on the justification aspects of arguments rather than the inferential and developed a model of these ‘real world’ argument based on facts, conclusions, warrants, backing and qualifier entities.

Using Toulmin’s model from evidence one can draw a conclusion, as long as it is warranted. Said warrant being possibly supported by additional backing, and possibly contingent upon some qualifying statement. Importantly one of the qualifier elements in practical arguments is what Toulmin called a ‘rebuttal’, that is some form of legitimate constraint that may be placed on the conclusion drawn, we’ll get to why that’s important in a second.

Toulmin Argumentation Example

You see Toulmin developed his model so that one could actually analyse an argument, that is argument in the verb sense of, ‘we are having a safety argument’. Formal safety arguments in safety cases however are inherently advocacy positions, and the rebuttal part of Toulmin’s model finds no part in them. In the noun world of safety cases, argument is used in the sense of, ‘there is the 12 volume safety argument on the shelf’, and if the object is to produce something rather than discuss then there’s no need for a claim and rebuttal pairing is there?

In fact you won’t find an explicit rebuttal form in either GSN or CAE as far as I am aware, it seem that the very ‘idea’ of rebuttal has been pruned from the language of both. Of course it’s hard to express a concept if you don’t have the word for it, nice little example of how language form can control the conversation. Language is power so they say.

 

Well it sounded reasonable…

One of the things that’s concerned me for a while is the potentially malign narrative power of a published safety case. For those unfamiliar with the term, a safety case can be defined as a structured argument supported by a body of evidence that provides a compelling, comprehensible and valid case that a system is safe for a given application in a given environment. And I have not yet read a safety case that didn’t purport to be exactly that.

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The development of safety cases for complex safety critical systems

So what is a safety case? The term has achieved an almost quasi-religious status amongst safety practitioners, with it’s fair share of true believers and heretics. But if you’ve been given the job of preparing or reviewing a safety case what’s the next step?

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Recently there’s been some robust discussion over on the Safety Critical Mail List at York regarding the utility of safety cases and performance based safety standards (as exemplified by the UK safety case regime) versus more prescriptive design standards (as exemplified by the aerospace industry FAR regulations). To provide one UK regulator’s perspective here’s a presentation by Taf Powell, Director of the Offshore Division of Health and Safety Executive’s Hazardous Industries Directorate, UK, on the state of safety cases in the UK offshore industry circa 2005. Of course his talk was well before the 2010 Deepwater Horizon disaster.

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