A Review of the Fundamentals on Process Capability, Process Performance, and Process Sigma, and an Introduction to Process Sigma Split

There are several indices for quality control, like Capability and Process Performance indices, within the scope of design and process optimization. Given the amount of existing literature produced in the last decades, and its level of complexity reached so far, this article aims to work as an essential digest of Process Capability and Process Performance indices, in relation also to Process Sigma (or Sigma Level), introduced within Lean Six Sigma. Particularly, the article provides a clear review of the fundamental concepts and applications of the abovementioned indices to calculate and evaluate process performance and process yield. Moreover, this article offers an overview of the features of each metric, specifying the main differences and defining hypotheses of their applicability in process performance evaluation. The article further clarifies the different methods to calculate Process Sigma (for centered and off-center processes with and without 1.5 shift). Besides outlining the essentials, reaching out also to a more operative audience in manufacturing and service industry, the novelty of this paper is also to suggest a rapid method to standardize statistical language and to confront either similar or competing processes. In fact, the authors have devised a new index named “Capability Difference”, to obtain a new method to calculate Process Sigma through “Process Sigma Split”, which allows to isolate the current performance from future performance, for a clearer evaluation of the process. This research, its results and insights, might further studies, and lead scientists to develop new, improved, and practical performance dashboards.