## 2 Research Context
### Modularity Metrics  
The literature presents a considerable number of modular design measures. These measures have been categorized into eight groups. Each of these groups of modular design measures is divided into subgroups to clarify their composition.  
Group 1 concerns measures aimed at determining a level of similarity or standardization at the level of parts, modules, or interfaces. Group 2 gathers measures that assess the level of interaction between components and the level of connectivity of interfaces by the number of interfaces between parts within modules, coupling between modules, or the level of exchange between modules. Group 3 focuses on measures that evaluate the degree of customization and variety of products. Group 4 assesses the size of modular systems and allows for the evaluation of the complexity of modular systems. Group 5 delves into evaluating the degree of modular design by the efficiency of modules or the functional independence between modules. Group 6 encompasses all measures of costs and the profitability of modularity. Group 7 includes all costs associated with the manufacturing department. Group 8 gathers measures that allow for comparisons between platforms. Table 3 presents the groupings of measures as well as the associated authors.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Group** & **Subgroup** & **Authors** \\ \hline \multirow{6}{*}{Group 1:} & Similarity Indexes & [19] \\ \cline{2-3}  & Components Commonality & [13; 21; 29] \\ \cline{2-3}  & Commonality Index & [13] \\ \cline{2-3}  & \multirow{2}{*}{Optimal Commonality} & [13] \\ \cline{2-3}  & & Optimal Commonality & [13] \\ \cline{2-3}  & \multirow{2}{*}{Component Sharing} & [13] \\ \cline{2-3}  & & Commonality Cost Advantage & [13] \\ \cline{2-3}  & \multirow{2}{*}{Interface Commonality} & [13] \\ \cline{2-3}  & & \\ \hline \multirow{6}{*}{Group 2:} & Component Connectivity & [5; 13; 29; 49] \\ \cline{2-3}  & \multirow{2}{*}{Coupling Concept} & [29; 49] \\ \cline{2-3}  & Interface & Flow Analysis & [36] \\ \cline{2-3}  & \multirow{2}{*}{Substitutability} & [29] \\ \hline \multirow{6}{*}{Group 3:} & Variety Index & [13] \\ \cline{2-3}  & Degree of Customization & [3] \\ \cline{2-3}  & Product Volume & [3] \\ \cline{2-3}  & \multirow{2}{*}{Customer Needs Rating} & [13] \\ \cline{2-3}  & \multirow{2}{*}{Differentiation Level} & [13] \\ \cline{2-3}  & \multirow{2}{*}{Variety and Differentiation Cost} & [13] \\ \cline{2-3}  & Product Variety Index & [13] \\ \hline \multirow{6}{*}{Group 4:} & DSM Size and & [5] \\ \cline{2-3}  & Number of Modules & [13; 27] \\ \cline{2-3}  & \multirow{2}{*}{Interface Number} & [13] \\ \cline{2-3}  & \multirow{2}{*}{Minimal Description Length} & [5] \\ \hline \multirow{6}{*}{Group 5:} & Modular Grouping Index & [28] \\ \cline{2-3}  & Functional Coupling & [13; 29] \\ \cline{2-3}  & Modularity Index & [5; 13] \\ \cline{2-3}  & \multirow{2}{*}{Degree of Modularity (Integral vs. Modular)} & [5] \\ \cline{2-3}  & \multirow{2}{*}{Function to Component Ratio} & [13] \\ \cline{2-3}  & Composition of Components & [13] \\ \cline{2-3}  & Granularity Level & [13] \\ \hline \multirow{6}{*}{Group 6:} & Discounted Cash Flow (DCF) & [13] \\ \cline{2-3}  & Flexibility Value & [13] \\ \cline{2-3}  & Business Costs & Profitability & [13] \\ \cline{2-3}  & Product Cost & [27] \\ \cline{2-3}  & Product Lifecycle Costs & [13] \\ \hline \hline \end{tabular}
\end{table}
Table 3: Modular metric groups.  
A thorough examination of modular design measures reveals their diversity. These groups of methods have different objectives. This diversity highlights the complexity of choosing tools and approaches without a clear guide.