What is the relationship between process stability and process capability?
Nothing and everything. Though they are not directly linked, statistician and SPC expert Steven Wachs cautions that without evidence of process stability, capability data is useless. Process Stability and Process Capability are both extremely important aspects of any manufacturing process. Often the concepts behind process stability and process capability and the relationship between them are misunderstood. This article attempts to clarify both ideas and the relationship between them. Defining Process Stability and Process Capability Process Stability refers to the consistency of the process with respect to important process characteristics such as the average value of a key dimension or the variation in that key dimension. If the process behaves consistently over time, then we say that the process is stable or in control. The graphic below illustrates a stable process. The process distribution remains consistent over time. The next graphic illustrates an unstable process. The process
