## 3 Cyber-Physical Risk Assessment Approach
These characteristics reflect the potential of a threat source to exploit the vulnerabilities considered in the cyber-physical attack scenarios generated in Activity 2. The overall assessment of a threat source is computed as the weighted average of the level of threat source characteristics. In Activity 5 the types of physical impact (X\({}_{\text{PI}}\)) are assessed: _User safety_, _Economic losses_, _Social and Political impact_, which can arise from the attack scenarios generated in Activity 2. Assessment of physical impact can be accomplished by considering each type of physical impact individually, resulting in a different risk level for each type of physical impact. Alternatively, physical impacts can be assessed jointly, for example, when a successful attack scenario can lead to both casualties and economic losses [32]. Activity 4 depends on the outcome of Activity 3. For example, if the exploitation of a cyber vulnerability has high attack complexity (X\({}_{\text{AC}}\)) then the importance of the knowledge of the threat source (W\({}_{\text{KN}}\)) should also be high. Activity 5 also depends on the outcomes of Activities 3-4. For example, if a vulnerability has a higher attack vector and it is exploited by a threat source that has high knowledge (X\({}_{\text{KN}}\)) and terrorism experience (X\({}_{\text{Te}}\)), it can potentially lead to a high physical impact (X\({}_{\text{PI}}\)). \begin{table}
\begin{tabular}{c c c} \hline \hline
**Level** & **Decision Scale Scores, X\({}_{i}\)** & **Importance Index Scores, W\({}_{i}\)** \\ \hline Very Low & 0–1 & 0–0.20 \\ Low & 2–4 & 0.21–0.40 \\ Medium & 5–7 & 0.41–0.60 \\ High & 8–9 & 0.61–0.80 \\ Very High & 10 & 0.81–1.0 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Levels and scores of decision scale and importance index. In Activity 6 assessors compute the cyber-physical risk following Equation (1), based on the outcomes of Activities 3-5. The cyber-physical risk is computed as the product of the overall assessment of vulnerability, threat source and impact. The numerical result of Equation (1) allows identifying a corresponding risk level as shown in Table 2.  
The use of different ranges of levels and scores enables a qualitative (i.e., Low) and quantitative (i.e., 0-1) guide for stakeholders to aid the computation of risk, following the structure of NIST SP800-30 [25], which has been widely used by security engineers. \[ \begin{split}\text{Risk}=&\frac{\ W_{\text{AV} \times\text{X}_{\text{AV}}}+W_{\text{AV}}\times\text{X}_{\text{AC}}+W_{\text{ PI}}\times\text{X}_{\text{QI}}+W_{\text{PR}}\times\text{X}_{\text{PR}}+W_{\text{JO}} \times\text{X}_{\text{RQ}}}{\text{W}_{\text{AV}}+\text{W}_{\text{AC}}+W_{ \text{UI}}+W_{\text{PR}}+\text{W}_{\text{IO}}}\\ &\times\frac{W_{\text{EN}}\times\text{X}_{\text{EN}}+W_{\text{ BE}}\times\text{X}_{\text{BE}}+W_{\text{PR}}\times\text{X}_{\text{PS}}+W_{\text{PR}} \times\text{X}_{\text{TE}}}{W_{\text{KN}}+W_{\text{PR}}+W_{\text{PS}}+W_{\text {PR}}}\times\text{X}_{\text{TE}}}\times\ X_{\text{PI}}\end{split}  \tag{1}\]  
The symbols of X\({}_{i}\), W\({}_{i}\) in Equation (1), indicate the level and the importance of _Attack vector_ (X\({}_{\text{AV}}\), W\({}_{\text{AV}}\)), _Attack complexity_ (X\({}_{\text{AC}}\), W\({}_{\text{AC}}\)), _User interaction_ (X\({}_{\text{UI}}\), W\({}_{\text{UI}}\)), _Privileges required_ (X\({}_{\text{PR}}\), W\({}_{\text{PR}}\)), _Interoperability_ (X\({}_{\text{IO}}\), W\({}_{\text{IO}}\)), _Knowledge_ (X\({}_{\text{KN}}\), W\({}_{\text{KN}}\)), _Resources_ (X\({}_{\text{RE}}\), W\({}_{\text{RE}}\)), _Psychology_ (X\({}_{\text{PS}}\), W\({}_{\text{PS}}\)), _and Terroism experience_ (X\({}_{\text{Te}}\), W\({}_{\text{Te}}\)) and type of physical impact (X\({}_{\text{PI}}\)). In order to expand the established calculation of risk from the assessment of generic terms such as vulnerability, threat source and impact [50], Equation (1) was developed.