## 2 Materials and Methods
As a response, we propose to define a classification scheme to provide an improved understanding of the consistency impairments with respect to the causal relations expressed in the FMEA documents. As such, it is critical to firstly identify the concept classes that logically should be represented by the data (i.e., matching content interpretation by domain experts). In many cases, the concept classes are domain-specific and typically cannot be inferred from the data alone; they need to be established with the help of domain knowledge. To this end, we propose a set of rules to govern the identification of the classes:  
* Concepts classes need to be completely separable;
* Concepts classes need to allow for the assessment of the causal relations consistency between concepts;
* Concepts classes need to be aligned with domain experts' perception of the cells' short text content. For our case study on FMEA documents, the identification of individual concept classes is rooted in actual production lines and based upon the knowledge of experienced domain experts. They were interviewed in order to establish our concept classification scheme, which forms the base for the FMEA documents consistency improvement methods. While our work is focused on causal relations found in FMEA documents of the semiconductor manufacturing industry, other domains also require domain-specific classification schemes, also taking expert knowledge and its formalization into consideration. First, on a high level, in the semiconductor manufacturing company under study, FMEA documents can be split into multiple types depending on the respective scope of the FMEA. One can distinguish between Process FMEA documents (P_FMEA) and Unit Process FMEA documents (UP_FMEA). An individual P_FMEA contains multiple causal chains, each of which might be associated with numerous processes used in a product production line. UP_FMEAs include causal chains only belonging to single processes, which in turn could be related to multiple products' production lines. The expected concepts described in the P_FMEA documents are different from those expected to be described in UP_FMEA documents. In P_FMEA documents, one can expect concepts _physical_ characteristics' deviation of the products' structures, which causes concepts describing _functional_ characteristics' deviation of the product. Many reasons could cause these physical characteristics' deviation. However, due to the high degree of manufacturing automation and the highly controlled environment (i.e., the typical manufacturing conditions in the semiconductor manufacturing industry), the most relevant causes stem from the so-called unit process deviation. In the UP_FMEA documents, it is expected to find concepts describing the so-called unit process key parameters deviation that causes the unit process deviation (which is also described in P_FMEA documents). Additionally, in the UP_FMEA documents, the root causes of the unit process key parameters deviation follow the 5M (Man, Machine, Material, Method, Measurement) risk-management model similar to the Ishikawa causal diagram. To connect the two types of FMEA documents, experts aim to link unit process cause in the P_FMEA document to the respective unit process effect of UP_FMEA. The linking concept (i.e., the unit process deviation) is abbreviated as UP-C/E. Domain experts further explained that the product functional characteristics' deviations are caused by deviations in physical characteristics of the product structures, while the reversed direction is not valid. To summarize, leveraging the information acquired from the discussions and multiple rounds of interviews with domain experts, we are finally able to distinguish five distinctive concepts classes: Functional, Physical, UP-C/E, Parametric, and 5M. The concept classes and their causal chains are depicted in Figure 3. To readers who are familiar with measurement data typically found in a modern semiconductor manufacturing FAB, these measurements types are also in line with the proposed concept classes.