Definitions - What Exactly is a Risk?

Creating a definition sounds as thought it ought to be easy. It isn’t for many reasons - some of these are not so much technical as the process by which consensus is reached and the need to get consensus. For example the need to get consensus might mean that at times a weaker definition escapes because it was too difficult to get consensus with a tighter one.

Why do we care? Well there is a particular and a more general reason. The more general one is that the graphic blocks we use to represent the real world things have definitions and therefore the architect is supposed to select the most appropriate block to represent the real world thing based on the description. We can’t just choose anything otherwise we end up “head-modelling” where the verbal description we provide is not supported by the semantics of the model we’ve created (the model in our head is not the one on paper). If the description is wrong it might not be the right block to use (you wouldn’t represent ‘tank’ with a ‘tree’).

The particular reason is that we’ve a working group in TRAK looking to see if and how it is possible to extend TRAK to enable it to be used to address typical safety-related and security-related concerns. One of the starting points is therefore a review of general literature and particularly standards to identify the potential concepts or entities likely to be needed. In doing so we’ve found some potential problems with definitions.

A candidate entity is risk. What is a risk?

IEC 61508:2010

combination of the probability of occurrence of harm and the severity of that harm

MIL STD 882E

Mishap Risk. An expression of the impact and possibility of a mishap in terms of potential mishap severity and probability of occurrence.

NIST

The net mission/business impact considering (1) the likelihood that a particular threat source will exploit, or trigger, a particular information system vulnerability and (2) the resulting impact if this should occur.

There is a common thread. Many other standards also have very similar forms of definition. None of these, however, defines what a risk actually is The analogy is defining force as the product of mass and acceleration - it tells us nothing of what force is. None of the above are therefore definitions of risk they just indicate how we might derive a metric for it. One of the principles in defining something has to be that the definition is independent of other variables or an implementation. In the above if risk didn’t involve probability of occurrence it would mean that the concept of risk itself had changed which isn’t true.

My dictionary provides:

a possibility of harm or damage

IEC 61508:2010 defines a Hazard:

potential source of harm [Guide 51 ISO/IEC:1990]..

’ which is fine but then in the note that follows it states ‘….for example, release of a toxic substance…’ which looks to be a hazardous event not a hazard.

All of this means that it is harder and takes longer than it should do to analyse and form a view of a pragmatic compromise because you have to examine every word and be selective in what you choose to accept and what you choose to reject. You cannot blindly assume that any standard is correct since it is as much the product of gaining consensus as it is the technical content. You have to be a skeptical enquirer and constantly challenge. Too often folks put such committees on pedestals and don’t stop and think.

External Links

• IEC 61508-4:2010 Functional Safety of Electrical/Electronic/ Programmable Electronic Safety-related Systems. Definitions and Abbreviations. June 2010
• National Institute Standards Technology. Special Publication 800-27 Rev A. Engineering Principles for Information Technology Security (A Baseline for Achieving Security), Revision A. June 2004
• Department of Defense. MIL STD 882E. Standard Practice for System Safety. 11th May 2012.
• Sourceforge. TRAK Viewpoints. Discussion Forums. Working Groups. Safety & Security

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