A process is considered stable, or in a state of statistical control, when the only sources of variation present are common causes. Common cause variation is the inherent, random, and statistically predictable "noise" within a process that is performing as designed. The absence of special (or assignable) causes of variation—which are unpredictable and arise from specific, identifiable circumstances—is the defining characteristic of a stable process. Control charts are used to monitor a process and distinguish between these two types of variation.

This statement is not false. The presence of special causes of variation is what defines an unstable or "out of control" process. A process exhibiting only common cause variation is, by definition, stable and predictable within its natural limits.

1. Montgomery, D. C. (2019). Introduction to Statistical Quality Control (8th ed.). Wiley.

Section 4.1, Basic Principles, p. 166: "A process that is operating with only chance causes [common causes] of variation present is said to be in statistical control. A process that is operating in the presence of assignable causes [special causes] is said to be out of control." This text is a foundational academic resource in quality engineering and statistical process control.

2. Oakland, J. S. (2014). Statistical Process Control (7th ed.). Routledge.

Chapter 3, Process Understanding, p. 39: "When the variation in a process is due only to chance (common) causes, the process is said to be stable or in a state of statistical control... The performance of a stable process is predictable." This peer-reviewed academic textbook directly equates a process with only common causes of variation to being stable.

DOI: https://doi.org/10.4324/9781315775332

3. MIT OpenCourseWare. (2004). 16.842 Fundamentals of Systems Engineering, Lecture 19: Quality, and Statistical Process Control. Massachusetts Institute of Technology.

Slide 14, "Statistical Process Control": The lecture notes explicitly state, "A process is in a state of 'statistical control' if it is operating with only common causes of variation." This material from a reputable university courseware directly supports the statement.