ASQ - Statistics Division

Six Sigma, Measurement Systems, and the Hidden Factory – Mini Paper Appearing in Spring 2003 Newsletter

Abstract: Measurement systems play an important role in any quality improvement initiative including a Six Sigma project. If correctly implemented, measurement systems can help determine the current state of a process and provide information leading to breakthrough performance; however, ineffective measurement systems can contribute to the overall hidden factory. This hidden factory is quantified by a calculation of a Cost of Poor Quality (COPQ). For example, ineffective measurement systems can lead to additional processing costs and loss of product value through unnecessary downgrading. The paper defines how the measurement system is a part of the hidden factory. The paper comments on the measurement indices %GR&R and P/T ratio, highlights the usage of Minitab, and shows examples of measurement systems in industrial applications. The paper gives suggestions for improving measurement systems. The key result from this work is to think of the measurement system as a process. Represent it as a box with inputs and outputs. Use a structured approach such as Six Sigma to improve the performance of both the measurement system and the overall system. As processes attain Six-Sigma performance they begin to rely less on sampling and more upon leveraging the few influential X variables, which are determined through multi-vari studies and Design of Experiments (DOE), which in turn are based on acceptable measurement analysis.

Keywords: Cost of poor quality (COPQ) - Gage repeatability and reproducibility (R&R) analysis - Precision to tolerance (P/T) ration - X-bar and R chart

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