## 2 Methodology, Fundamentals, and Materials
### Lean Production System and Improvement Strategies  
Lean is a process improvement methodology used to deliver products and services better, faster, and at lower cost [12]. Lean manufacturing is based on the principles and processes of the Toyota production system (TPS) [32]. The TPS developed as a result of the market requiring the production of small quantities of many varieties under low demand conditions after World War II to test whether Japanese car manufacturers could compete with the mass production systems established in Europe and the United States [33]. Toyota produced automobiles with less inventory, human effort, investment, and defects and introduced a greater variety of products [34]. Lean manufacturing creates more value for customers by eliminating activities that do not add value to the product or service. Lean production can be considered as a philosophy and a set of tools and practices for the continuous improvement of production operations that are of the highest quality, the lowest cost, and the shortest lead time [32]. Personnel has a fundamental role in the process of developing a lean strategy and can be essential for the success of lean techniques, as the early adopters in Japan discovered [35]. Lean production is one of the best approaches for promoting business excellence through continuous improvement [36], and lean management remains the most popular approaches to operational excellence across industries. The lean approach has evolved over time and been embraced by other industries and in other cultures [37]. The standard elements of a global production system are quality management and robust processes, logistics and production control, work organization, employee orientation, standardization and visual management, continuous improvement, and product and process development. Various other strategies have emerged in the literature to cope with changing market requirements and to optimize production systems:  
* Lean manufacturing places two management goals above any other: continuous improvement and the constant need for waste reduction [16] to reduce costs [6]. LM seeks to continuously decrease costs, defects, and inventories while increasing product variety [38]. The principles of LM eliminate waste in every function within a company; although, at the very beginning, it had been focused on production [6]. To achieve this goal, Toyota applied the scientific method to all levels of the workforce, ensuring that people shared a common goal and vision of what the production system should be [39]. Without using lean methods and improving processes, the processes could become unstable. Stability is defined as the capability to produce consistent results over time. Instability is the result of process variability. The first step in creating lean processes is to achieve a basic level of process stability that can be consistently maintained [40]. * Six sigma is a management philosophy developed by Motorola in 1986 that requires setting extremely high objectives, collecting data, and analyzing results to reduce defects in products and services. Today, it is used in many industrial sectors. It has also been referred to as a systematic approach to the quality improvement of process outputs with variability minimization in manufacturing and business processes [15]. Six sigma helps to manage the challenge of complexities in products and processes by minimizing the risk of low quality by controlling variability. After inception of TQM in the early 1980s, six sigma arrived as an element of TQM that was seen an evolution in quality management [19]. Six sigma is a systematic methodology aimed at operational excellence through continuous process improvements that have been successfully implemented worldwide for over 20 years, producing significant improvements to the profitability of many large and small organizations [41]. Six sigma has been called the best-known approach to process improvement.