# Systemic Evaluation of PSS in the Early Concept Phase Using Graph-Based Reasoning
## 4 Solution Approach
### PSS Graph Elicitation and Analytic Reasoning (GEAR) Method  
Creating a mental model of the PSS developers regarding the PSS concept as a graph model and using it for reasoning analysis in order to be able to evaluate a PSS concept requires the completion of various steps. The graph elicitation and analytic reasoning (GEAR) method is based on the PSS abstraction framework and explains this procedure using the four steps shown in Figure 12.  
Step 1 - Preparation: Definition of scale and scope of the evaluation in preparation for elicitation of a priori domain knowledge  
Step 2 - Elicitation of graph model supporting EVC evaluation: Elicitation of a priori domain knowledge and creation of graph-model according to PSS abstraction framework  
Step 3 - Reasoning Analysis: Graph-based reasoning analysing expected level of EVC achievement and impact of knowledge gaps within the PSS concept  
Step 4: - **valuation and discussion of potential measures:** Evaluation of reasoning analysis results and derivation of measures to improve the PSS concept in understanding and merit  
Step 1: Preparation  
The GEAR-method requires the EVCs and their metrics as input to be evaluated, as defined by engineering management requesting the evaluation. If multiple EVCs are to be compared, an evaluation function that prioritizes the achievement of the EVCs must also be provided by management. The time frame for the evaluation must also be determined in collaboration with management. For example, the EVC of _localizability of technical systems in the field_ requires a different evaluation logic depending on the time horizon. Localizing technical systems with a focus solely on the operational phase requires fewer functions than if localizability is to be ensured until the end-of-life phase. Since different operational scenarios of the PSS concept are crucial depending on the EVCs, the scale and scope of the PSS concept regarding the functions to be considered and the PSS elements involved must also be established. This is done by the experts conducting the method. The definitions of the functions and elements of the PSS concept to be considered form the semantic  
Figure 11: PSS concept variation layer with deterministic assignment of possible combinations of PSS element variations. Figure 12: Steps of the GEAR method. framework for gathering a priori knowledge from domain experts in step 2 of the GEAR method. The scope of consideration may change during the course of the method, e.g., if during the discussions, domain experts realize that features or behaviors of PSS elements that were not considered in step 1 are relevant. As a result of step 1, there are clear definitions of the EVCs to be evaluated, along with their metrics (and a scoring function for multiple EVCs). In addition, EVC-relevant operational contexts for the PSS concept are described in terms of functions and the function-fulfilling elements of the PSS concept. This preparation step serves as the foundation for gathering the causal relationships in step 2. Step 2: Elicitation of a graph model supporting EVC evaluation  
In the concept phase of a product service system (PSS), a priori knowledge is often implicit due to the high level of abstraction and lack of formalized models. In step two of the GEAR method, this knowledge is elicited and formalized through workshops with domain experts from relevant fields (e.g., hardware, service, and legal). These experts bring different perspectives, focusing on influences specific to their domain. The knowledge elicitation process relies on the reference framework from step one, which includes PSS elements and the functional structure.