## 2 Research Background and Related Work
### Agile-Oriented Frameworks for Hardware Development  
**SAFe**. The Scaled Agile Framework (SAFe), created by [PERSON], is a set of organizational and workflow patterns for implementing and scaling practices and techniques at the enterprise level in accordance with lean principles [38]. SAFe is a body of knowledge that incorporates frameworks such as Scrum, Extreme Programming, Kanban and Lean at the team level [39]. It advocates four levels: Value stream, Program, Portfolio and Team level [40]. The principal values of SAFe, which are key to SAFe's effectiveness, concern: alignment, built-in quality, transparency, and program execution [41]. In this line of thinking, the Scaled Agile organization provides six universal principles, outlining how SAFe applies to hardware development [42]:  
1. Organize Around Value;
2. Assume Variability and Preserve Options;
3. Build Incrementally, Integrate Frequently;
4. Design for Change;
5. Perform Work in Small Batches;
6. Build Continuous Integration for Hardware Development. While SAFe is built on top of the agile methodology, harnessing its advantages, in particular, is based on ten immutable, underlying Lean-Agile principles; the above six principles have been arbitrarily selected and reformulated in the context of the hardware domain. Nevertheless, there is open space for creativity and innovation, since, as stated by the authors, \"they can be used to guide hardware engineers to create and adopt their own best practices\" [42]. **Scrum for Hardware**. Scrum for Hardware (or Scrum for HW) is a framework that combines Scrum with modular architecture and Lean/XP practices [43], customizing the agile practices and techniques for hardware and system design. The Scrum in Hardware Guide defines 8 areas to consider before, during and after implementation, namely:  
1. Uncertainty in Hardware Projects;
2. Stubs and Mock-Ups;
3. Modularize;
4. Continuously Integrate;
5. Interface Design;
6. Test and Data-Driven Development;
7. Team;
8. Working Product at the End of Each Sprint. In general, the guide can be used by any organization, small or large, regardless of its field of activity, structure, or organization. **Hybrid Stage-Gate**. The Stage-Gate (SG) process is a conceptual and operational road map for moving a new-product project from idea to launch and beyond [44]. In the simplest format, SG \"consists of two major components:  
1. a series of stages, where the project team undertakes the work, obtains the needed information, and does the subsequent data integration and analysis, followed by;
2. gates, where go/kill decisions are made to continue to invest in the project\" [44]. An overview of the general Stage-Gate system is depicted in Figure 1.  
Stage-Gate is a macro model, designed to support selecting the projects to be followed by the model, and once selected, to map out the key stages, best practices, management techniques, roles, and responsibilities. On the other hand, there is an agile approach that brings micromanagement practices and techniques to the table, which are proven to foster adaptability, encourage creativity, and improve productivity. [PERSON] and [PERSON] introduced a hybrid approach, which incorporates Agile methods for physical product development [45], by integrating them with the SG system, and then utilized them for manufactured or physical product development (hardware). In other words, they argue that \"Agile is normally applied as the project management method within the stages in Stage-Gate\", while \"the stages remain, and Agile is applied within some stages\" [45]. Moreover, Scrum is a favored methodology since it has been indicated as \"the most appropriate for hardware development\" for all case studies uncovered so far in industry. **Modified Agile for Hardware Development (MAHD)**.