Stability has many different definitions. This article will discuss the concept in terms of Lean Six SIgma and process stability. Stability will also be contrasted with process capability.
Overview: What is stability?
Simply speaking, it is the tendency of a process to perform in a consistent manner over time. Your process may be consistently good or consistently bad but, at least it is consistent and predictable. You can say your process is stable if it is operating only with common cause variation
Stability is one of the characteristics of your process that is determined by using a control chart. A control chart is designed to distinguish between common and special cause variation. By collecting process data over time, the control chart will indicate whether the process is in a steady state condition and is stable and predictable or is not random or predictable and therefore not stable.
Here is a graphic illustrating a stable and unstable process:
Stability only indicates whether the process is consistent or not. Capability describes whether your stable process is capable of meeting your customer specs or internal expectations and targets. Since process capability looks at future performance, the process must be stable before you can assess capability.
Now we can answer the question why your doctor may keep you waiting in the waiting room or why you need to fast before taking a blood test. Your bodily functions are a process. To understand what your process is doing, you must start with a stable process. Waiting in the waiting room will give you a chance to calm your nerves and start to attain a steady state condition for your body. LIkewise, fasting for 24 hours allows any special cause variation to be reduced and your process to stabilize so the test can accurately measure your process.
An industry example of stability
Below you see two control charts. The first one shows signs of special cause variation and would be defined as an unstable process. After making some process improvements, the second control chart shows only common cause variation and stability.
Frequently Asked Questions (FAQ) about stability
Is there a statistical definition of stability?
How do you test for stability?
A control chart is a statistical tool for monitoring a process over time. If your control chart indicates that only common cause variation exists, you can claim your process is stable.
Is stability and capability the same thing?
No. Stability measures the consistency of your process over time but does not assign any goodness or badness to your process. Capability will tell you whether your stable process is capable of meeting your customer specifications or other performance indicators.