Defining and categorising systems. A key concept in the definition and conceptual model of cascading effects is described above is “system”, which needs further elaboration. In general, a system can be defined as “a regularly interacting or interdependent group of items forming a unified whole” 1. However, the choice of what can be regarded as “a system” in a particular context is subjective, and the choice of system boundaries will clearly affect which phenomena that will be considered as relevant. Hence, this choice is extremely important as it largely determines the focus of the project. The choice of system boundaries can always be criticised in one way or another since it is always possible to draw the boundaries differently. This is especially the case when dealing with larger, more complex systems. Xxxxxxxx, for example, argues that the boundaries of a complex system can never be clearly defined because: “different descriptions of a complex system decompose the system in different ways, [and therefore] the knowledge gained by any description is always relative to the perspective from which the description was made” (Xxxxxxxx, 2005, p. 258). Hence, there is no single or true way of describing systems boundaries. We take a pragmatic approach and instead of defining clear-cut system boundaries, we define system categories for which it is easier to argue for inclusion or exclusion of real-life systems. Another benefit of defining system categories is that they can be employed in all case studies and therefore facilitate subsequent aggregated analyses. The aim of the method, which was described in Chapter 3.3.1, is that it should capture the description of propagations of effects between distinctly different societal sectors, rather than focusing on smaller-scale cascading effects within particular systems, such as within the electric transmission system. In order to develop a system categorization in line with this aim, the area of critical infrastructure protection can be used as a source of inspiration, as this is an area that is concerned with identifying sectors and infrastructures that are essential for the functioning of society. Several different system categorizations have been suggested in the literature. In Figure 3.2 a number of categorisations of critical infrastructures used in Europe and the U.S. are compared. In the United States, the critical infrastructure categories mentioned in official reports and directives have varied over time (1983-2003); ranging from 3 to 26 di...
