Assessing Process Capability: A Case Study

Abstract

Research efforts in the area of process capability have largely been devoted to finding a better process capability index (PCI) and to a lesser extent on the stochastic behavior of the estimated PCIs [1], [2]. Much of this development has gone unused for many reasons including a) a plethora of indices, b) interpretation, c) software support, d) standards and e) dissemination. The addition of the indices appears to have had little impact on the practitioners. Cp and Cpk (including C_pl and C_pu) [3] continue to be the most heavily used indices with C_pm [4] and C_pmk [5] occurring occasionally. The addition of stochastic assessments for estimated PCIs is a positive development, however statistical developments have frequently lacked background knowledge and implementation ease, hindering use by practitioners. A case study from the printing industry will be used to draw attention to areas impacting the practical use of PCIs. Concepts including a) establishing effective tolerance limits, b) the ongoing assessment and interpretation of PCIs and c) ongoing improvement will be presented. We will use the case to a) illustrate a strategy followed by practitioners using PCIs as a quality management tool, b) to draw attention to gaps that exist in the practical use of PCIs, c) to illustrate how some of these difficulties were overcome and d) to highlight research areas in the practical use of PCIs. A variety of quality tools including flowcharts/process documentation, control charts, process capability indices and experimental design are illustrated throughout the manuscript. Data used in the analyses has been included where permitted. Although drawn from the printing industry, the tools used, lessons learned and generic achievements are applicable to the wider area of product design and manufacturing, particularly where customers have unique requirements for a common product.

Key Words:

• Design specifications
• process capability
• process capability chart
• tolerances

Additional information

Notes on contributors

Smiley W. Cheng

Smiley Cheng is Professor and Head of the Department of Statistics at the University of Manitoba. He has served as Associate Head and Acting Head of the Department in the past. He was the President of the International Chinese Statistical Association and the Managing Editor of Statistica Sinica. He is currently the Associate Editor or member of Editorial Board of five international statistics journals. Dr. Cheng is heavily involved in the research in statistical quality control (SQC), statistical inference, order statistics, and lottery. He is a Senior Member of American Society for Quality, a member of Statistical Society of Canada, American Statistical Association, International Chinese Statistical Association, and an Elected Member of the International Statistical Institute.

Bartholomew P. K. Leung

Bartholomew P. K. Leung is an Assistant Professor in the Department of Applied Mathematics at Hong Kong Polytechnic University, Kowloon, Hong Kong. He received his Ph.D. in Statistics from the University of Manitoba, Winnipeg, Manitoba, Canada. His research interests include the fields of quality assurance, applications of quality, loss functions and their associated properties. Professor Leung has published articles in those areas while also been working closely with industry in promotion of quality and the use of quality tools.

Fred Spiring

Fred Spiring is a Senior Member of the American Society of Quality, Adjunct Professor in the Department of Statistics, the University of Manitoba, and a retired Director of Pollard Banknote Ltd in Winnipeg, Manitoba, Canada. His research interests include the fields of quality assurance, process capability, process control, and loss functions. Professor Spiring has worked with industry in the promotion of quality and quality tools. As well, he has worked within the educational system initiating and serving as the first Director of Quality for The University of Manitoba and in the creation of the University of Western Ontario’s Quality Centre.