## 2 Bibliometric Analysis
### Bibliometric Analysis Process  
In order to identify relevant research fronts in the area of equipment maintenance systems, [13] provided a bibliometric analysis process that can be followed, consisting of ten steps. Based on this process, the Knowledge Discovery in Databases (KDD) process, the workflow given in [29], and the statements given by an expert interview, a general-purpose process of bibliometric analysis is provided in the following (see Figure 8).  
At the start of a bibliometric analysis, its goals have to be set (1), i.e., the type of results that are expected must be defined, e.g., finding research fronts and knowledge bases. Next, a dataset has to be collected (2). Therefore, one or many data sources need to be selected. Frequently used bibliographic data providers are Web of Science, Google Scholar (scholar.google.com (accessed on 6 May 2021)), Scopus (scopus.com (accessed on 6 May 2021)), Microsoft Academic (academic.microsoft.com (accessed on 6 May 2021)), and Science Direct (sciencedirect.com (accessed on 6 May 2021)). This step also includes the definition of query strings that are used to find publications in the databases.  
Figure 7: Example research fronts network.  
Then, the raw data needs to be extracted from these databases. Afterward, the data are preprocessed (3). This usually encompasses data cleansing, e.g., removing duplicates and incomplete records, enriching the dataset with data from further information sources, and, in the case that multiple data sources are used, merging the raw data. Based on the defined goals, some bibliometric algorithms are selected and executed to compute (4) and visualize (5) the results. In the end, the results must be interpreted (6).  
As in the KDD process, it is possible and may be necessary to go back a step at any time in the process. For example, when visualizing research fronts with the Fruchterman-Reingold algorithm, the parameter of the number of iterations has to be set. If the first parameter selection leads to an unsatisfying visualization, it may be adjusted and rerun. Furthermore, having finished a bibliometric analysis, it may be done again at some point in the future to keep track of the field of research.  
In the introduction, two important aspects were discussed. First, we delineated how bibliometric analyses essentially work and what aspects, including limitations, must be considered when creating a technical solution. As shown, for this work, we focus on the calculation of hybrid research fronts (important part of bibliometric networks), and therefore the important aspects are conveyed. Based on this, we discussed how hybrid research fronts are supported by ambalytics.  
This means that the essential procedure to calculate them is presented, see Figure 4. Then, we show how the procedure is incorporated by ambalytics, see Figure 8. As for many other domains [30], the shown principle can be considered as the standard operating procedure (SOP) in this context. SOPs, in turn, are a powerful instrument to see in a comparable way how parts of a system work. In the following, we demonstrate how ambalytics technically implements the SOP of hybrid research fronts.