Process Capability Indices Based on Median Absolute Deviation
Kayode S. Adekeye
Process capability is the performance of a process under normal and in-control conditions. Its indices are to measure the inherent variability of a process and thus to reflect its performance. To calculate the capability indices for key characteristics, most industries normally assume that the distribution of their process output is normal. However, in practice, most key quality characteristics do fails the normality test and thus the accuracy of normal based process capability indices becomes doubtful and hence they cannot really reflect the performance of a process. This paper proposes the determination of the common process capability indices based on the median absolute deviation. The median absolute deviation is a robust estimate of variability when the sample data are non- normal or are skewed. Real process data and simulated process data from heavily skewed distributions are presented to demonstrate the application of the proposed indices and the results were compared with the existing indices for non-normal and skewed process data.
Full Text: PDF