Tool support for safety cases can be considered in three broad categories:
Decision support and elicitation tools. These allow one to expose expose the thinking behind the argument, advise on how to construct a case, and assist in reading and review. They are considered in more detail below.
Tools to generate evidence. These provide the evidence that support the safety case argument. They include safety analysis tools (fault trees, FMECAs), tools for collecting and analysing field experience, static analysis, test and proof tools.
Safety Management System Infrastructure support. In this category there are the tools for configuration management and traceability such as Requirements Engineering support tools and Hazard Logs.
Although safety cases are increasingly accepted and mandated for assuring critical systems, the traditional means of production ? word processed documents with in-line graphics ? has a number of shortcomings. Traditional applications have to be severely stretched for safety case development and the resulting documents are often cumbersome, and can be difficult to construct and review. Moreover, the structure of the safety argument itself is often lost in the volume of paper produced.
Toulmin developed a conceptual framework and graphical notation for representing the structure of an argument in the 1950s. Toulmin  makes a distinction between "claim or conclusion whose merits we are seeking to establish" and "the facts we appeal to as a foundation for the claim". Together with the notion of a "warrant" that the facts indeed support the claim, Toulmin developed the following basic notation:
Toulmin's graphical argumentation motif
A generic graphical argument using a "claims-arguments-evidence" structuring motif
Integrating narrative and graphical notation into a hypertext argumentation approach
There are deficiencies in just using plain narrative, or a purely graphical notation. Pure narrative is critiqued earlier ? such documents can be long, unstructured, and often do not bring out the implicit argument. Pure graphical notation can demonstrate links between argument sections and differentiate between different types of argument components, but without narrative there is no "meat" against which the soundness of the argument may be judged.
We propose that a hypertext argument is like software, in that it is "enacted" through reading the notation in conjunction with the narrative. At a macro level, a user "reads" and expands the graphical argument with narrative according to the structure, thereby recreating an overall narrative/set of utterances for the hypertext argument (e.g. "this node is the evidence for the 'design diversity' argument which supports the main safety claim"). The graphical notation allows the user to focus on particular structures and follow threads of supporting evidence. Similarly to software there can be bugs in the argument (e.g. a claim may be floating or unsupported, or a piece of evidence may be invalid). At a micro level, there is a need for standard narrative so that authors can explicate any necessary details about the argument, situate it in context, and make use of any existing narrative. This approach is implemented in the Adelard Safety Case Editor (ASCE)  and has been used on real safety cases with many thousands of nodes.
This page is based on Luke Emmet & George Cleland, Graphical Notations, Narratives and Persuasion: a Pliant Systems Approach to Hypertext Tool Design,in Proceedings of ACM Hypertext 2002 (HT'02), June 11-15, 2002, College Park, Maryland, USA.
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