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1. Introduction
Six Sigma approach is defined as:
[…] a systematic and organized technique to improve the strategic process, new product and development of service that focus on scientific and statistical methods to make considerable reductions in customer determined defect rates (Linderman et al., 2003).
The initial concept of Six Sigma was basically developed and executed by Motorola in 1987 so that they could aim to achieve a difficult target of 3.4-ppm defects, namely, 3.4 defects per million opportunities (Schroeder et al., 2008). Later on, plenty of companies including General Electric, Honeywell, Sony, Caterpillar and Johnson Controls have performed Six Sigma and gained considerable benefits (Zu et al., 2008). Not only Six Sigma has been prevalently incorporated into manufacturing and service industries because of the positive effects on quality improvement and cost reduction but also it has gained extreme popularity and acceptance among academic community (Kaushik and Khanduja, 2009). Hong and Goh (2003) explored the applicability and suitability of the Six Sigma framework for performance improvement of software industries. In addition, they discussed some common misconception and practical challenges for implementing Six Sigma in software projects. One of the most critical issues faced by manufacturers today is how to deliver products or materials fast, at low cost and good quality (Zahraee, 2016). Garg et al. (2004) formulated a mathematical programming problem to apply design of Six Sigma into supply chains. The objective was to determine optimum allocation of lead-time variabilities and inventories to improve the delivery performance. Sokovic et al. (2005) applied Six Sigma methodology in process design, in which they used some process improvement tools such as process map and the cause and effect matrix. Li et al. (2006) performed computer-aided engineering-based Six Sigma robust optimization procedure to improve design quality and removed the effects of uncertainties in sheet metal forming process. Mahesh et al. (2006) used Six Sigma for benchmarking of rapid prototyping and manufacturing processes and recognized the best performance to decrease variability. Su and Chou (2008) developed an approach to generate the Six Sigma projects according to the organization’s business strategic policies and voice of customer (VOC). Then, they characterized projects as Green Belt, Black Belt or other types of project by usage of analytical hierarchy process...