Sigma metric revisited: True known mistakes.

Abdurrahman Coskun,Mustafa Serteser,Ibrahim Ünsal

doi:10.11613/bm.2019.010902

Abstract

Six Sigma methodology has been used successfully in industry since the mid-1980s. Unfortunately, the same success has not been achieved in laboratory medicine. In this case, although the multidisciplinary structure of laboratory medicine is an important factor, the concept and statistical principles of Six Sigma have not been transferred correctly from industry to laboratory medicine. Furthermore, the performance of instruments and methods used in laboratory medicine is calculated by a modified equation that produces a value lower than the actual level. This causes unnecessary, increasing pressure on manufacturers in the market. We concluded that accurate implementation of the sigma metric in laboratory medicine is essential to protect both manufacturers by calculating the actual performance level of instruments, and patients by calculating the actual error rates.

Highlights

Six Sigma methodology is the latest version of total quality management and has been widely used in industry since the mid-1980s
To calculate the defects per million opportunities (DPMO) corresponding to sigma metric (SM), we find the area under the curve (AUC) from the lower tolerance limit (LTL) to upper tolerance limit (UTL) from the z table and use Eq 4
If we use Eq 5 in the performance calculations of analysers, methods, reagents, and other instruments, we will obtain performance values significantly lower than the actual levels. This results from two biases: the bias directly included in the equation, and the 1.5 SD shift accepted as the natural bias

Summary

Introduction

Six Sigma methodology is the latest version of total quality management and has been widely used in industry since the mid-1980s. We aimed 1) to explain the statistical techniques of how engineers calculate short and long-term SM and error rates in industry and business, and 2) to show the defects of the SM equation used in laboratory medicine. The DPMO corresponding to SM is derived from the area under the curve (AUC) restricted by LTL and UTL as calculated below (Eq 3): This equation gives AUC but is rather complex to be used in daily practice. To calculate the DPMO corresponding to 5 SM, we find the AUC restricted by - 5 to 5 from z table and use Eq 4 as given below: AUC from − 5 to 0 = 0.4999997133. Due to the 1.5 SD shift, the limit of the left tail of the curve is − 6.5 (−1.5 - 5) and the limit of the right tail of the curve is 3.5 (5 -1.5) (Figure 1)

SD Shift

Conclusion