Process capability analysis for non-normal distribution with lower specification limit

Abstract

Process capability analysis is an important element of any quality improvement initiative. However, estimating process capability is often problematic when it comes to non-normal distributions since the conventional methods sometimes give misleading results. In this paper, a new method for estimating process capability of non-normal distribution with only lower specification limit is proposed. The proposed method only considers the left tail of the distribution rather than taking all of the data points. For estimating the process capability, it employs least squares technique and normal approximation to the selected observations from the left tail. Here, the proposed method is only tested on lognormal distribution. Simulations and real-world data analysis are used for verification and validation purpose, respectively. An easy and practical guideline is also developed.