This is the second part of the series (part 1 here) providing some background on the way we perform risk assessments. It can be seen as a direct continuation of the last post; today I cover the method of estimation and the scale & calculation formula used.
1.1 Method of Estimation
Again, two main approaches exist:
- Qualitative estimation which uses a scale of qualifying attributes (e.g. Low, Medium, High) to describe the magnitude of each of the contributing factors listed above. [ISO 27005, p. 14] states that qualitative estimation may be used
- As an initial screening activity to identify risks that require more detailed analysis.
- Where this kind of analysis is appropriate for decisions.
- Where the numerical data or resources are inadequate for a quantitative estimation.
As the latter is pretty much always the case for information security risks, in the infosec space usually qualitative estimation can be found. A sample qualitative scale (1–5, mapping to “very low” to “very high”) for the vulnerability factor will be provided in the next part of this series.
- Quantitative estimation which uses a scale with numerical values (rather than the descriptive scales used in qualitative estimation) for impact and likelihood, using data from a variety of sources. [ISO 27005, p. 14] states that “quantitative estimation in most cases uses historical incident data, providing the advantage that it can be related directly to the information security objectives and concerns of the organization. A disadvantage is the lack of such data on new risks or information security weaknesses. A disadvantage of the quantitative approach may occur where factual, auditable data is not available thus creating an illusion of worth and accuracy of the risk assessment.”
1.2 Scale & Calculation Formula Used
Each of the contributing factors (that are likelihood, vulnerability [factor] and impact) will be rated on a scale from 1 (“very low”) to 5 (“very high”). Experience shows that other scales either are not granular enough (as is the case for the scale “1–3”) or lead to endless discussions if too granular (as is the case for the scale “1–10”).
Most (qualitative) approaches use a “1–5” scale.
It should be noted that usually all values (for likelihood, vulnerability and impact) are mapped to concrete definitions; examples to be provided in the next part. Furthermore the “impact value” will not be split into “subvalues” for different security objectives (like individual values for “impact on availability”, “impact on confidentiality” and so on) in order to preserve the efficiency of the overall approach.
To get the resulting risk, all values will be multiplied (which is the most common way of calculating risks anyway). It should be noted that there are some objections with regard to this approach which, again, will be discussed in the next part.
The values themselves should be discussed by a group of experts (appropriate to the exercise-to-be-performed) and should not be assigned by a single person. The usefulness and credibility of the whole approach heavily depends on the credibility and expertise of the people participating in an exercise!
Wherever possible some lines of reasoning should be given for each value assigned, for documentation and future use purposes.
 [ISO 27000], section 220.127.116.11 provides a good overview.
 Models with quantitative estimation don’t use a “vulnerability factor” (as this one usually can’t be expressed in a quantitative way).
 And so does the example 2 (section E.2.2) of [ISO27005] which can be compared to the methodology described here.
Feel free to get back to us with any comments, criticism, case studies, whatever. Thanks,