Integrating six sigma and theory of constraints for continuous improvement: a case study Ike Ehie and Chwen Sheu Department of Management, Kansas State University, Manhattan, Kansas, USA Abstract Purpose – To investigate the potential of combining six sigma (SS) and theory of constraints (TOC) to improve production system performance. Design/methodology/approach – Based on the literature, this paper proposes an integrated TOC/SS framework and applies this framework to an axle manufacturing company to improve its gear-cutting operation. Findings – The results of the case study indicate that the company benefited tremendously from its emphasis on global improvement guided by the TOC concept. Managers were able to select a continuous improvement (CI) project that had greater impact on bottom line performance. On the other hand, SS provided various statistical tools and engineering techniques (such as value analysis, Pareto diagram and control charts) for defining the specific process to be improved, analyzing the root causes, and designing actions for making improvement. Research limitations/implications – For the future effort of integrating TOC with SS and other CI techniques, we suggest the incorporation of a TOC methodology, thinking process (TP). Practical implications – Overall, the integration of SS and TOC provides managers with an excellent platform for two CI techniques to work together and create great synergy. Originality/value – This study adds to the CI literature and practice by demonstrating the value of a method of combining TOC and SS. Keywords Six sigma, Continuous improvement, Quality management Paper type Case study Various continuous improvement (CI) strategies have been developed and applied to improve manufacturing system performance. This paper studies the combination of two distinct strategies, six sigma (SS) and theory of constraints (TOC), for improving manufacturing system performance. We first review these two strategies and the processes involved in each strategy. The possibility of integrating them for improving manufacturing system performance is examined. We then apply this integrated approach to a manufacturing company in the midwestern USA that specializes in the production of axles. The effectiveness of this integrated approach is examined, and finally recommendations of applying this integrated approach are presented. SS and TOC SS is basically a business process improvement methodology in which sigma represents a statistical measure of variability in the process. Under conditions of normality the SS represents 2 parts per billion (PPB); however, considering the long-term process drift of 1.5 sigma espoused by Motorola (1990), a process operating The Emerald Research Register for this journal is available at The current issue and full text archive of this journal is available at www.emeraldinsight.com/researchregister www.emeraldinsight.com/1741-038X.htm JMTM 16,5 542 Received April 2003 Revised November 2003 Accepted March 2004 Journal of Manufacturing Technology Management Vol. 16 No. 5, 2005 pp. 542-553 q Emerald Group Publishing Limited 1741-038X DOI 10.1108/17410380510600518