The Goal Structuring Notation (GSN)

This entry describes the basics of the notation which can be used for writing arguments. A list of web resources on GSN is provided at the end of the entry. GSN has been used for writing arguments in:

  • Safety
  • Dependability
  • Security

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NEWS: The draft Goal Structuring Notation Standard is available for comment. http://www.goalstructuringnotation.info/ As the drafting committee’s website says: “The Standard has two intended functions. Firstly, it seeks to provide a comprehensive, authoritative definition of the Goal Structuring Notation (GSN). Secondly, it aims to provide clear guidance on the current best practice in the use of the notation for those concerned with the development and evaluation of engineering arguments – argument owners, readers, authors and approvers.” This is a great time to get involved to ensure that the standard meets the intentions of the drafting committee.

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The Goal Structuring Notation (GSN) is a graphical notation for presenting the structure of (safety) arguments. Primarily it acts as a communication means to describe how a particular claims has been shown to be true by means of evidence. The technique has been used within the Nuclear, Defence, Aerospace and Rail industries. Within an assurance case (be it for safety, dependability, security, etc) the argument demonstrates how the set of evidence items combine together to demonstrate the top claim (e.g. that the system is acceptably safe to operate in a particular operating environment).  Items of evidence can include (but are not limited to) process information, product information, qualitative data, quantitative data, subjective information, service history, analysis, testing, verification and validation.

GSN explicitly represents the individual elements of an argument (requirements, claims, evidence and context) and (perhaps more significantly) the relationships that exist between these elements (i.e. how individual requirements are supported by specific claims, how claims are supported by evidence and the assumed context that is defined for the argument). The principal symbols of the notation are shown below:

GSN Notation


The principal purpose of a goal structure is to show how goals (claims about the system) are successively broken down into (“solved by”) sub-goals until a point is reached where claims can be supported by direct reference to available evidence (solutions). As part of this decomposition, using the notation it is also possible to make clear the argument strategies adopted (e.g. adopting a quantitative or qualitative approach), the rationale for the approach (assumptions, justifications) and the context in which goals are stated (e.g. the system scope or the assumed operational role). An example safety argument in GSN is shown below:

GSN Example

As well as using GSN for developing individual safety arguments, GSN Patterns can be used to capture successful argument approaches that may be used across many different arguments.

Note: People often use the term Goal Structured Notation or Goal Structuring Notation however the actual name of the notation is Goal Structuring Notation (it is a notation for structuring goals!

GSN is currently not standardised, however work is in progress to produce a GSN Standard. A similar notation to GSN is Claims-Argument-Evidence (CAE), however this is even less clearly defined.

Useful Resources

There is currently no published reference book on GSN.

The following two papers provide a good introduction to the notation:

Systematic Approach to Safety Case Management T P Kelly
in Proceedings of SAE 2004 World Congress, Detroit, March 2004 (Proceedings published by the Society for Automotive Engineers)

The Goal Structuring Notation – A Safety Argument Notation T P Kelly, R A Weaver
in Proceedings of the Dependable Systems and Networks 2004 Workshop on Assurance Cases, July 2004

The GSN Club webpages have public resources describing GSN, links to relevant publications and GSN tools. The club run by Origin Consulting aims:

  • To provide a forum for members to exchange best practice experience in use of GSN on their projects
  • To build up resources (e.g. Safety Case Patterns) to help those using GSN on their projects
  • To provide a forum for members to hear about the latest developments and extensions to GSN
  • To provide an impartial forum for members to hear about the latest offerings in tool support for GS

Appendix E of the The Yellow Book contains an explanation of the notation, a description of a methodology for its use (this is important to know and often difficult to find), a case study and some useful references.  The document is aimed at the Railway Industry.

Eurocontrol has published a Safety Case Development Manual, which has a definition of GSN in it.  However there is a warning associated with this document – THIS DOCUMENT DESCRIBES A NON STANDARD GSN NOTATION.  For some reason, the authors have chosen to rename “goals” as “arguments”.  This can be very confusing for those familiar with the Claims-Argument-Evidence (CAE) notation (similar to GSN) in which the argument symbol is equivalent to a strategy in GSN.  The manual is primarily aimed at the European ATM industry.

Adelard offer a Safety Case Development Manual which will be sent to you via email when you register for it at their website.

Can’t find the information you want? Please leave a comment telling me what you were looking for and I will add it – how this resource works.

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One Response to “The Goal Structuring Notation (GSN)”

  1. Sisebuto Fitzhume García Says:

    I’ve found this website very interesting and useful. In fact, the links have given me very important information related to the development of a Safety Case.

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