Comparison of the GD and MTR approaches for estimating proportion of non-conformance

This paper compares our proposed method of using the modified tolerance region (MTR) to the generalised distance (GD) approach for estimating the probability of non-conforming product. For correlated quality characteristics, the tolerance region needs to be redefined so that the model is more stringent in identifying potentially poor product quality, as perceived by customers. The paper shows that the MTR approach is more effective in estimating the proportion of non-conformance (PNC) for correlated quality characteristics as compared to the GD approach. We also compare the multivariate capability index computed using the PNC values obtained from both the GD and MTR methods. The estimation of PNC assists not only in the evaluation of process performance and capability but also in the quantification of cost of poor quality.