Workingbear has already spent a good amount of time explaining what Cp (Precision) and Ck (Accuracy, sometimes called Ca) mean, and what each represents. We’ve also learned that using Cp or Ck alone can give an incomplete picture when it comes to quality control. So now, Workingbear is going to introduce the combined process capability index — Cpk — which is widely used today as a standard measure in statistical process control (SPC).
Ck, which some people now refer to as Ca (Accuracy), represents how close a process output is to the target value. Using a target-shooting example: if someone fires six bullets and all land near the bullseye, the Ck value is good (meaning low), because the average result is close to the center of the specification — a direct hit.
Cp (short for Precision) is a measure of how tightly a process is centered and how consistently it produces results. Using a target-shooting analogy, if someone fires six bullets and they all hit the same tight spot, that means the process has high Cp. The tighter the grouping, the higher the Cp value.
Cpk is commonly used as an overall indicator of a factory’s manufacturing process capability. However, it can also be applied across different industries to measure the ability to meet a target. A popular analogy is how well you can hit the bullseye when shooting arrows at a target.
Of course, the higher the Cpk value, the better. But Cpk is actually made up of two components: Cp and Ck.
Many of you might already know what a fishbone diagram is. But do you know how to actually draw one? And what should you watch out for when creating one?
As the name suggests, the diagram looks like the skeleton of a fish—that’s where it gets its nickname. Its formal name is Cause-and-Effect Analysis Diagram, because it’s used to trace a problem (effect) back to its possible causes.
