Process stability and instability

Process stability

Variation remains consistent, predictable, and measurable when all factors of variation remain the same.

Process instability

Process instability occurs when exceptional events happen, such as introducing new equipment, a new operator, or change in raw material suppliers. These changes increase variation and make the process unpredictable.

These changes are considered special causes that can increase or decrease process variability. When a special cause occurs, the level of variation increases, indicating that the process is no longer operating predictably and is therefore considered an unstable process.