## 2 Materials and Methods  
In order to address the issue introduced in the previous section and to answer the aforementioned questions, a systematic literature review was conducted. This section firstly describes the methodology employed in such a review, and secondly, offers a brief bibliometric analysis of the results. The literature review was carried out in four stages--see Figure 2: database search, screening, eligibility and literature analysis. Figure 1: Key concepts used for the systematic literature review. Figure 2: Search process and results, adapted from PRISMA [10]. The databases used for the initial stage were SCOPUS (Elsevier) and Web of Science, and only included relevant publications belonging to the following fields: Manufacturing engineering, Industrial engineering, Generalist engineering, Operations and management science. Since the topic under study is the conjunction of several broad subjects, we decided to conduct a systematic literature review that specifically targets their intersections. The six key concepts that were used are Assembly, Mass customization, Key Performance Indicator (KPI), Lean manufacturing, Industry 4.0 and Operator. These concepts were chosen for the search because they are the key ideas in the posed research questions-\"Key Performance Indicators\" being used for measuring improvement. The following keywords were used to perform the database search: (1) Lean: _Lean manufacturing_, _Lean production_; (2) Mass customization: _mass customization_, _mass customization_; (3) Industry 4.0: _Industry 4.0_, _Industrie 4.0_, _smart factories_; (4) KPI: _\"KPI\"_, _Key Performance Indicator_; (5) Assembly: _assembly_; (6) Operator: _operator_, _people_, _person_. The keywords were used for Title, Author Keyword and Keyword Plus (in WOS), except for KPI, which was also searched for in the Abstract field. From these six key concepts, 15 search groups were defined by intersecting each possible combination of two concepts, as shown in Table 1. Duplicates were removed at this point, resulting in 1026 publications identified. The publications resulting from this search were then screened--based on title, abstract, publication and year--to assess which of them met the inclusion and exclusion criteria shown in Table 2, resulting in 741 records being excluded and 285 articles being included. \begin{table}
\begin{tabular}{l c c c} \hline \hline
**Search Group** & **Publications WOS** & **Publications SCOPUS** &
\begin{tabular}{c} **Publications Identified** \\ **after Duplicates Removed** \\ \end{tabular} \\ \hline Assembly and mass & & & & \\ customization & 58 & 52 & 97 \\ Assembly and KPI & 20 & 19 & 33 \\ Assembly and Lean & 81 & 106 & 168 \\ Assembly and Industry 4.0 & 47 & 10 & 55 \\ Assembly and operator & 83 & 196 & 268 \\ Industry 4.0 and Lean & 48 & 8 & 55 \\ Industry 4.0 and operator & 33 & 16 & 45 \\ Industry 4.0 and mass & 17 & 2 & 19 \\ customization & Industry 4.0 and KPI & 11 & 2 & 12 \\ Lean and mass & 14 & 19 & 32 \\ customization & 31 & 58 & 74 \\ Lean and operator & 10 & 33 & 40 \\ Operator and mass Mass & 4 & 15 & 15 \\ Customisation & 13 & 98 & 108 \\ Mass customization and KPI & 4 & 3 & 5 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Search groups created by the intersection of each pair of key concepts and number of publications found. \begin{table}
\begin{tabular}{l l} \hline \hline
**Inclusion Criteria** & **Exclusion Criteria** \\ \hline Peer-reviewed publications & Book chapters \\ \hline Recent: published in 2010 or later & \
\begin{tabular}{l} Regarding construction, continuous production (e.g., petrochemical), \\ energy efficiency \\ \end{tabular} \\ \hline Language: publications in English &
\begin{tabular}{l} Regarding product design \\ \end{tabular} \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Eligibility and exclusion criteria. Finally, the 285 articles were reviewed within each one of the 15 search groups and assessed for eligibility, resulting in 51 articles being excluded because they were not relevant to the key concept being analyzed.