Six Sigma definitions from different PhD dissertations

“The strategy that provides companies with a series of interventions and statistical tools that can lead to breakthrough profitability and quantum gains in quality, whether a company’s products are goods or services.” (Harry, M., and Schroeder, R. (2006). Six sigma: the breakthrough management strategy revolutionizing the world’s top corporations, Crown Business.)

The word Sigma is a statistical term that measures how far a given process deviates from the mean, which represents perfection. (Isa, Mohsen F. Mohamed, “Quality Modeling And Improvement Of University Facilities Services Using Six-Sigma – A Case Study On Wayne State University Fpm Services” (2013).Wayne State University Dissertations. Paper 5.) Variation in processes is defined as any quantifiable difference between individual measurements; such differences can be classified as being due to common causes (random), or special causes (assignable) Beady, S. (Fall 2005). “Waste On War.” Master in Quality Assurance, California State University Dominguez Hills..

The Six-Sigma methodology is designed to provide a systematic way of applying statistical

tools in the context of process improvements in any organization. This is done by the application of the DMAIC methodology.  The DMAIC framework entails the identification and elimination of the sources of variation in a process; improving and sustaining performance with well-executed control plans; and promoting one process improvement language for all members of an organization to utilize. (Isa, Mohsen F. Mohamed, “Quality Modeling And Improvement Of University Facilities Services Using Six-Sigma – A Case Study On Wayne State University Fpm Services” (2013).Wayne State University Dissertations. Paper 5.)

General Electric Corporation, one of the big early implementers of Six-Sigma, emphasized that Six-Sigma is a highly disciplined process that helps us focus on developing and delivering near-perfect products and services.  (Define, Measure, Analyze, Improve, and Control) Antony, J. (2006). “Six sigma for service processes.” Business Process Management Journal, 12(2), 234-248.

Six-Sigma methodology emphasizes listening to the voice of the customer in order to identify the customer’s needs and requirements and converting them into specifications in the design of the service or production that can be monitored and measured Lee, K.-L. (2002). “Critical Success Factors of Six Sigma Implementation and the Impact on Operations Performance.” Doctor of Engineering Dissertation, Cleveland State University.


Six Sigma is a technique used to evaluate business processes. With the evaluation of the business processes, Statistical Process Control tools are used to make sound unbiased decisions that benefit the business, its customers, associates, and shareholders McCarty, T., Daniels, L. Bremer, M., Gupta, P. (2005). The Six Sigma Black Belt Handbook. New York: McGraw-Hill.


Six sigma was created as a continuous quality improvement technique, but today it is significantly different from total quality management (TQM) approach of the 1980s. The key differences between six sigma and TQM are listed on Table 2.1 Barney, M. (2002). Motorola’s second generation. Six Sigma Forum Magazine, 1 (3).

Table 2.1 Differences between six sigma and total quality management (Barney, 2002)

Six Sigma Total Quality Management
Executive ownership Self-directed work teams
Business strategy implementation system Quality programme
Truly cross functional Largely within a single function
Focused training with verifiable return on investment No mass training in statistics and quality
Business results oriented Quality oriented

Six sigma can be defined as a process improvement methodology that aims to increase business performance through a solid and accurate business focus (Savolainen & Haikonen, 2007). Savolainen, T. & Haikonen, A. (2007). Dynamics of organizational learning and continuous improvement in six sigma implementation. The TQM Magazine, 19 (1), 6-17. Also, Pyzdek defines six sigma as the application of the scientific method to the design and operation of management systems and business processes, while enabling employees to deliver the greatest value to the customers (Pyzdek, 2003). Pyzdek, T. (2003). The six sigma handbook: A complete guide for green belts, black belts and managers at all levels. USA: McGraw-Hill. So it can be specified as a systematic approach for accomplishing continuous process improvement.  Senem VAHAPLAR, A Study On Six Sigma Applications For Developing Process Performance, Dokuz Eylül University Graduate School of Natural and Applied Sciences, Page 3 

 Table shows the relation between sigma levels and DPMO (Defect for million opportunities) values. It is clear that DPMO and sigma levels are indirectly proportional. Sigma levels increase as DPMO values decrease. The lower the DPMO value, the higher the sigma level and this shows an absolute improvement in the processes. This improvement can be monitored with the success rate column in Table 2.2. As DPMO value decrease and therefore sigma level increase, the success of the process increases.

DPMO values, sigma levels and the corresponding success rates

DPMO Success rate Sigma level
841,000 16.00000 % 0.5
691,000 31.00000 % 1.0
500,000 50.00000 % 1.5
309,000 69.10000 % 2.0
159,000 84.10000 % 2.5
66,800 93.32000 % 3.0
22,800 97.72000 % 3.5
6,210 99.37900 % 4.0
1,350 99.86500 % 4.5
233 99.97670 % 5.0
32 99.99680 % 5.5
3.4 99.99966 % 6.0

Senem VAHAPLAR, A Study On Six Sigma Applications For Developing Process Performance, Dokuz Eylül University Graduate School of Natural and Applied Sciences, Page 5 


 The Six Sigma program contains several elements

± A business context: Six Sigma intends to help companies to survive in a competitive environment by creating cost savings, improving customer satisfaction, and improving organizational competence for innovation and continuous improvement.

± An organization structure: Six Sigma offers a management structure for organizing continuous improvement of routine tasks. Six Sigma prescribes that improvement is done in a project-by-project fashion.

± A methodology: Six Sigma offers a method for carrying out improvement projects.

These three elements will be elaborated in the next subsections. De Mast, J., Does, R.J.M.M. and De Koning, H. (2006). Lean Six Sigma for Service and Healthcare. Beaumont Quality Publications, Alphen aan den Rijn, The Netherlands.


Six Sigma methodology is a structured around understanding the customer needs, while identifying key processes linked to the customer needs. Statistical and quality tools are used to reduce variations in the key process which can sustain the process over time. (Rampersad, H.K. and El-Homsi, A. (2007) ‘TPS-Lean Six Sigma: Linking Human Capital to Lean Six’, Illustrated, IAP.)

According to Jing (2009) Six Sigma is an improvement methodology that is primarily aimed at improving process capability by reducing variation, variation is reduced by being consistent.

Jing, G.G. (2009) ‘A Lean Six Sigma BREAKTHROUGH’, Quality Progress, 42(5), 24-31.

Magnusson et al (2003) define Six Sigma as a business process that allows them to monitor

everyday business activities and design ways that minimize waste and number of resources which will improve their profitability, and all while increasing customer satisfaction. Magnusson, K., Kroslid, D. and Bergman, B. (2003), ‘Six Sigma – The Pragmatic Approach’, Lund, Studentlitteratur.

Antony (2006) discussing Six Sigma with reference to service processes is of the opinion that the term sigma is a measure indicating the deviation in the performance characteristic of a service from its mean performance. Antony, J. (2006) ‘Six Sigma for Service Processes’, Business Process Management Journal, 12(2), 234-248. Therefore, the basic goal of a Six Sigma strategy is to reduce variation within the tolerance or specification limits of a service performance characteristic.  Ian Fitzmaurice, The effects of implementing a Lean Six Sigma tool, Visual Process Controls, to improve month-end activities in a finance department An Empirical Case Study

The UK department for Trade and Industry while defining says; Six Sigma is: “A data driven method for achieving near perfect quality. Six Sigma analysis can focus on any element of production or service, and have a strong emphasis on statistical analysis in design, manufacturing and customer-oriented activities”

Motorola University while defining Six Sigma management system says : ‘’..The Six Sigma management system drives clarity around the business strategy and the metrics that most reflects success with that strategy. It provides the frame to prioritize resources of projects that will improve the metrics, and it leverages leaders who will manage the efforts for rapid, sustainable, and improved business results..”

GE defines Six Sigma as process…”a highly disciplined process that helps us focus on developing and delivering near perfect products and services.

Why ‘Sigma’? The word is a statistical term that measures how far a given process deviates from perfection. The central idea behind Six Sigma is that if can measure how many ‘defects’ you have in a process, you can systematically figure out how to eliminate that and get as close to zero’ 3.4 defects per million opportunities. An ‘opportunity’ is defined as a chance fore nonconformance, or not meeting the required specifications. This means we need to we need to be flawless in executing our key process”

Thus Six Sigma is a statistical concept that measures a process in terms of defects. Achieving six sigma means your processes are delivering only 3.4 defects per million opportunities (DPMO). Six Sigma is also a philosophy of managing that focuses on eliminating defects through practices that emphasize understanding, measuring and improving processes. In short, Six Sigma is several things:

i) A statistical basis of measurement: 3.4 defects per million opportunities

ii) A philosophy and a goal, as perfect as practically possible

iii) A methodology

iv) A symbol of quality

There are three key elements of quality: customer, process and employee.

Customers expect performance, reliability, competitive prices, on-time delivery, service and more. We have to delight our customer to ensure our market position.

Process: Process shall add significant value and produce output which exceed

customer’s expectation

Customer: Anyone Who Receives Product, Service, or Information

Opportunity: Every Chance to Do Something either “Right” or “Wrong”

Ian Fitzmaurice, The effects of implementing a Lean Six Sigma tool, Visual Process Controls, to improve month-end activities in a finance department An Empirical Case Study


Six Sigma is a customer-focused continuous improvement concept to achieve near perfect processes, products and services. This, in turn, can lead to waste reduction and greener industries. In other words, Six Sigma as a quality improvement approach is aimed at optimizing processes while reducing defects and costs as its key principles. Asadolahniajami, Anahita, “Applying the principles of Six sigma to environmental management systems : lessons learned from a case study” (2011). Theses and dissertations. Page 11.

Motorola describes three different concepts for Six Sigma: metric, methodology and management system; the last of which includes all three levels at the same time (Motorola, 2010).

Six Sigma as a metric refers to the statistical understanding of Six Sigma or the number of standard deviations away from the mean. The target of the metric concept is 3.4 measurements

outside of six standard deviations from the mean in every one million opportunities (Raisinghani et al., 2005). Raisinghani, M. S., Ette, H., Pierce, R., Cannon, G., & Daripaly, P. (2005). “Six Sigma:concepts, tools, and applications”. Industrial Management & Data System , Vol. 105 (No. 4), pp.491-505. There is a standard conversion table (Appendix 1) which is known as Z to DPMO (defects per million opportunities) conversion table that translates Sigma level into a corresponding number of defects. Appendix 1 provides detailed insight of the statistical meaning of Six Sigma.

As a Methodology, Six Sigma is a business improvement method that has two common forms inspired by the Deming cycle: DMAIC and DFSS. Define, Measure, Analyse, Improve, Control

(DMAIC) is the most well-known Six Sigma methodology. Design for Six Sigma (DFSS), also known as DMADV (Define, Measure, Analyze, Design, and Verify), is used for projects aimed

at creating new product or process designs (Williams, M. A., Bertels, T., & Dershin, H. (2001). Six Sigma: Pocket Guide. Lexington,Massachusetts: Rath & Strong)

The Six Sigma approach as a management system is all about helping managers identify the unknowns in their organization and take action to reduce errors and rework. Six Sigma translates that knowledge into opportunities for business growth Brue, G. (2002). Six Sigma for Managers. New York, London : McGraw-Hill Professional. In other words, Six Sigma measures success based on technical requirements established by customer and business targets.

Academicians started showing interest in Six Sigma from about 2000 onwards (Prabhushankar & Devadasan, 2008). There are many discrepancies in the literature on Six Sigma and researchers and practitioners have defined Six Sigma in different ways. Prabhushankar (2008) categorizes these definitions in three groups: statistics-based, management-oriented and holistic (both statistics-based and management-oriented definitions). In statistics-based definitions, the emphasis is more on the application of statistical techniques to reduce the process variation in reaching a performance level of 3.4 DPMO. Prabhushankar (2008) asserts “these definitions approach Six Sigma from statistical, probabilistic and quantitative points of view”. He adds “management-oriented definitions view Six Sigma as a philosophy and quality management system (QMS) to reduce variation, drive out wastes and meet the customer expectations through changing the culture of the organisation”. Prabhushankar (2008) concludes, holistic definitions describe Six Sigma as a quality management system and philosophy, while also emphasising the use of statistical and other problem-solving approaches.

Statistical Based

Six Sigma is a statistical measure of the performance of a process or product which is used as a quality control mechanism, that seeks to reduce defects or variations in a process to 3.4 per million opportunities thereby optimizing output and increasing customer satisfaction Kumi & Morrow (2006) Kumi, S., & Morrow, J. (2006). “Improving self service the six sigma way at Newcastle University Library”. Program: Electronic Library and Information Systems , Vol. 40 (No. 2), pp.123-136.

Six Sigma is a statistically-based quality improvement program, helps to improve business processes by reducing waste, by reducing costs resulting from poor quality and by improving the levels of efficiency and effectiveness of the processes Hensley & Dobie (2005) Hensley, R., & Dobie, K. (2005). “Assessing readiness for six sigma in a service setting”. Managing Service Quality , Vol. 15 (No. 1), pp.82–101.

Six Sigma is a disciplined method of using extremely rigorous data gathering and statistical analysis to pinpoint sources of errors and ways of eliminating them Harry & Schroeder (2000) Harry, M., & Schroeder, R. (2000). Six Sigma: The Breakthrough Management Strategy

Revolutionizing the World’s Top Corporations. New York, NY: Currency-Doubleday.

Management Oriented

Six Sigma is a business improvement strategy that seeks to find and eliminate causes of defects or mistakes in business processes by focusing on outputs that are of critical importance to customers Snee (2004) Snee, R. D. (2004). “Six-Sigma: the evolution of 100 years of business improvement methodology”. International Journal of Six Sigma and Competitive Advantage , Vol. 1 (No. 1),pp. 4-20.

Six Sigma is a break through strategy that combines improved metrics and a new management philosophy to significantly reduce defects, thereby strengthening a firm’s market position and

improving the profit line Hensley & Dobie (2005) Hensley, R., & Dobie, K. (2005). “Assessing readiness for six sigma in a service setting”. Managing Service Quality , Vol. 15 (No. 1), pp.82–101.

Six Sigma is a business improvement strategy that is used to improve profitability, drive out waste, reduce quality costs and improve the effectiveness and efficiency of all operations that meet or even exceed customers’ needs and expectations Bañuelas & Antony (2002) Bañuelas, R., & Antony, J. (2002). “Critical success factors for the successful implementation of six sigma projects in organizations”. The TQM Magazine , Vol. 14 (No. 2), pp.92–99.


Six Sigma is an organized and systematic method for strategic process improvement and new product and service development that relies on statistical methods and the scientific method to make dramatic reductions in the customer defined defect rates Bhuiyan & Baghel (2005) Bhuiyan, N., & Baghel, A. (2005). “An overview of continuous improvement: from the past to the present”. Management Decision , Vol. 43 ( No. 5), pp.761–771.

Six Sigma is a statistical term that roughly translates to only 3.4 defects or failures or mistakes/errors per million opportunities. The Six Sigma approach emphasizes in understanding and documenting the business process, developing metrics and hard data, and reducing process variation Makrymichalos et al. (2005) Makrymichalos, M., Antony, J., Antony, F., & Kumar, M. “Statistical thinking and its role for industrial engineers and managers in the 21st century”. Managerial Auditing Journal , Vol. 20 (No. 4), pp.354–363.

Six Sigma is a comprehensive program for managing a business that emphasizes an intelligent blending of the wisdom of the organization with proven statistical techniques to improve both the

efficiency and effectiveness of the organization in meeting customer needs Maleyeff &

Krayenvenger (2004) Prabhushankar, G., & Devadasan, S. (2008). “The origin, history and definition of Six Sigma: a literature review”. International Journal of Six Sigma and Competitive Advantage , Vol. 4 (No. 2), pp. 133-150.


One Response to Six Sigma definitions from different PhD dissertations

  1. Pingback: Six Sigma, Innovation and the Story of 3M | Joe Shaheen the HR Machine

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