# Systemic Evaluation of PSS in the Early Concept Phase Using Graph-Based Reasoning
## 4 Solution Approach
### Evaluation Logic Layer (_EVL_)
Importantly, the working principle itself is not represented as a node in the graph; rather, it governs the relationships between the properties of PSS elements, the life-cycle context, and their ability to fulfill purposes or generate risks. The causal chaining of working principles through direct pathways, such as the input/output relations of operands and the coupling of properties of PSS elements, and indirect pathways, such as changes in the life-cycle context, establish the causal structure necessary for achieving EVCs. EVCs are represented as utility nodes in the Bayesian network formalism that attribute value (scalar values) to respective states of a node. However, the focus of this solution approach does not lie in determining a utility function but, rather, in explaining  
Figure 4: Ontology explaining the semantic framework at the core of the evaluation layer. Dotted connections are not self-referencing but, instead, indicate dependencies on another working principle. causally how well the metric that the utility function refers to is achieved based on a causal understanding. The working principles depicted in the graph relate to various PSS life-cycle situations, encompassing different structural and temporal levels of granularity. The approach aims to capture PSS behavior by describing life-cycle factors, design element properties, and their purposes/risks using scenarios. A scenario, in general, describes a specific situation for a system of interest, incorporating elements, properties, and relationships to convey operational uses within a conceptual framework [67]. Scenarios communicate complex concepts, aiding in stakeholder alignment by highlighting necessary capabilities, stakeholders, and conditions to achieve objectives [29]. They are crucial in early development to formalize requirements and assess potential boundaries [68; 69], and they aid in hazard and risk assessments by outlining expected behavior [30; 70]. Effective scenarios are _useful_, detailing specific if-then questions; _coherent_, maintaining consistency and completeness; and _simple_, omitting unnecessary details [67]. Scenarios can be formal or informal, varying in abstraction, from high-level purpose descriptions to structured descriptions, depending on the development stage [71]. For the purpose of the solution approach, they should be as elaborate as possible to capture semantic meaning, i.e., they should capture the possible operating situations for a working principle in natural language but also must be formalized and reduced to a symbolic level in order to enable reasoning. Scenarios describing the PSS concept within its life-cycle context can relate to heterogeneous domains depending on the specific application and the EVC under consideration. These factors include the following:  
* _Technological factors_: Performance limits of specific technologies, the integration capability of the company, the ease of maintenance of a given design, etc. ;
* _Service-related factors_: Competence of service staff conducting maintenance, service coverage for a specific area, response times when failures are detected, etc. ;
* _Customer-specific factors_: Acceptance of an offering, user behavior, differences between user groups, etc. ;
* _Environmental factors_: Climatic conditions, availability of required space in the operational environment, integration with other stakeholders, etc. ;
* _Market-related factors_: Demand, prices, costs, etc. ;
* _Organizational factors_: Legal framework, necessary internal approvals, contracts regarding risk transfer, ability to develop required technologies, etc. In capturing scenarios within a product service system (PSS), two approaches to causality are used: the _procedural view_ and the _dependency-based view_[72]. These views differ in their representation of temporal dynamics and system relationships. The procedural  
Figure 5: Syntactic representation of semantic PSS concept abstraction as a graph.