On the gap between theory and practice of process capability studies

Abstract

Historically, a theoretical framework has been established to judge whether a process is capable or not. These studies, called process capability studies, have received both positive and negative comments. Both researchers and practitioners have indicated that the concept of process capability studies is often misused in practice. This article aims to identify and quantify this misuse, and when it is severe, try to provide some explanations of its origin. The survey presented is primarily based on questionnaires and carried out among 97 Swedish organisations where process capability studies are used regularly. It shows that there is often a gap between theory and practice concerning process capability studies. The article also provides some possible answers to why the gap exists. It has been found that the gap between theory and practice can be explained by management issues, practical problems, conservative personal attitudes and methodological aspects. Based on the results, some suggestions for successfully implementing and using process capability studies are presented.

Process capability analyses based on fuzzy measurements and fuzzy control charts

“…If a process is out of control, it has historically failed to display a reasonable degree of consistency and it is illogical to expect that it will spontaneously do so in the future. Therefore, a capability study monitoring an unstable process will only express the capability of the process at that very moment and nothing can be said about the capability of the process in the future (Deleryd, 1998). We know that the best tool to check whether or not the process is in statistical control is control charts.…”

Process capability indices for a regularly adjusted process

“…It is suggested that only data collected from a stable process can provide a true picture of its capability. Non-random patterns on a control chart suggest the presence of assignable-cause variations that will make the result of any capability index meaningless (Deleryd 1998). For processes with unknown trend patterns, traditional process capability indices should not be used.…”

Performance Evaluation Using Multivariate Non-Normal Process Capability

“…They reduce the dimension of the multivariate QCs using the covariance distance (CD). Many studies in the literature consider non-normal PCIs [8,11,[42][43][44][45][46][47][48][49][50]. Additionally, a few studies focus on multivariate non-normal process capabilities.…”