Confidence Interval based Quality Improvement for Non-normal Responses

Abstract

Robust parameter design is an effective tool to determine the optimal operating conditions of a system. Because of its practicability and usefulness, the widespread applications of robust design techniques provide major quality improvements. The usual assumptions of robust parameter design are that normally distributed experimental data and no contamination due to outliers. Optimizing an objective function under the normality assumption for a skewed data in dual-response modeling may result in misleading fit and operating conditions located far from the optimal values. This creates a chain of degradation in the production phase, e.g., poor quality products. This paper focuses on skewed experimental data. The proposed approach is constructed on the confidence interval of the process mean which makes the system median unbiased for the mean using the skewness information of the data.  The response modeling of the midpoint of the interval is proposed as a location performance response. The main advantages of the proposed approach are that it gives a robust solution due to the skewed structure of the experimental data distribution and does not need any transformation which causes any loss of information in estimation of the mean response. The procedure and the validity of the proposed approach are illustrated on a popular example, the printing process study

Keywords

• Confidence interval,Dual response surface,Non-normal data,Robust parameter design

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