Why Modern Process Capability Analysis Must Go Beyond Conventional Cpk
Process capability indices such as Cpk have long been the industry standard for evaluating manufacturing performance.
When a process achieves a Cpk of 1.33 or higher, manufacturers generally consider it capable of consistently meeting specification requirements.
But what if the process itself changes over time?
What if consecutive measurements are no longer statistically independent?
And what if the reported Cpk looks excellent—while production quality continues to deteriorate?
These situations are becoming increasingly common in modern manufacturing.
From high-speed machining and semiconductor fabrication to precision molding and automated assembly, production processes often exhibit time-dependent behavior that conventional capability analysis cannot fully explain.
In these cases, relying solely on traditional Cpk may lead to overly optimistic conclusions and incorrect engineering decisions.