## 4 Simulation Models and Sustainability: Characterization
### Hybrid Simulations (HS)  
Hybrid simulations (HS) find their beginning as early as the beginnings of simulation as a discipline, with attempts to bring together the continuous and discrete techniques [150]. HS is defined as combined use of two or more simulation models from the three main simulation approaches of DES, ABS, and SD. The classification of hybrid simulations based on discreteness and continuity has been conceived [151] along three categories, namely,  
* Type 1 models: DES and SD hybrid models which contain both continuous and discrete elements;
* Type 2 models: ABS and DES hybrid models which contain either continuous and discrete elements; \begin{table}
\begin{tabular}{p{42.7 pt} p{113.8 pt} p{113.8 pt}} \hline \hline
**Criteria** & **SD** & **Effects on Sustainability Assessment** \\ \hline Abstraction level & Meso-Macro & The data magnification and strategic application imply that the units of modeling in case of SD are constituted at the organization level, e.g., modeling organizational sustainability in an SC organization [32]. \\ \hline \multirow{2}{*}{Scope} & \multirow{2}{*}{Strategic} & Owing to the systemic level, the SD is used to simulate long term sustainability, e.g., sustainable supplier selection in the auto industry [148]. \\ \hline \multirow{2}{*}{Data} & \multirow{2}{*}{Qualitative} & Data sources are broadly drawn. The data are subjective and largely judgmental, allowing for client-based sustainable policy-making. Less costly measurements required for different model variables with data for the quantitative modeling [94]. \\ \hline \multirow{2}{*}{Use-case} & Policy making/ causality & SD models owing to their systemic and policy nature find use in addition to individual researchers, international agencies, government agencies in case of sustainability, e.g., sustainable development of regional industries [149]. \\ \hline \hline \end{tabular}
\end{table} 
Table 4: _Cont._  
Figure 15: Keyword network of the collected research dealing with SD modeling in SC and manufacturing. The three clusters show predominance of keywords including green logistics, environmental impacts, automotive industry, and energy efficiency, respectively. * Type 3 models: ABS+DES+SD models which combine type 1 and type 2 models. Although the use of HS proliferated during the 1960s and 1970s, this growth was curtailed owing to the lack of any computing support in form of any software. This all changed with the turn of the century when the OR problems became more and more complex and the growth of computing such as rise of AnyLogic software, HS modeling picked up pace. Unlike in the past, where the narrative was limited to continuous and discrete modeling, the HS modeling in the 21 st century focused on the interaction between operational levels (modeled using DES and ABS) and strategic levels (modeled using SD). The use of ABS is invoked in situations where behavioral aspects are sought to be modeled [152]. The methodology of hybridization between the three models has been formalized using a number of frameworks. A recent endeavor formalizes the hybridization of SD and DES through five major designs, namely, parallelism, sequential use, enrichment, interaction, and integration [75]. In the case of parallelism, the two modeling techniques are used in parallel, independently, such that both techniques working within the same boundaries, take an identical view of the situation. In the sequential approach, each model is used for a given purpose with very little overlap between the boundaries of the two. In the case of enrichment, one model (SD) forms the core of the technique while the other (DES) is used as an enhancement; both the models take the same view of the system with one being dominant throughout the process. In the interactive approach, the two models work independently but share the information with each other. This exchange takes place at a fixed time.