Cpk value interpretation - What is the significance of Cpk value of 1.33?

can somebody educate me how to interpret this.

I got a Cp value of 1.00 and a Cpk of 0.9. But all the X bar values are within UCL and LCL. Does this mean that the process is under control. If so, what is the significance of Cpk value of 1.33

raghuraman:

Cp = Rating of specification limit usage
Cpk = Rating of process centering

Even if all Xbar values are within the control limits one of the individuals in the samle size could be out of the control limit.

Therefore beacause the Xbars are within limits does not always mean the process is in control.

You have to ensure that the process is stable by removing special causes and ensure there are no runs or trends.

In the case of inherent causes you need to identify them and control depending upon what your specs or customer requirements are. A type of inherent cause would be tool wear.

From the limited data you provided, my guess is you are not in control because the values you provided lead me to Guess that your range is unstable which means a higher sigma and lower Cp/Cpk readings.


Any help from someone more experienced in the process?

ASD...

Al,

Your logic is right on target IMHO. Cp does not take into account the process centering. Once you look at Cpk you have to consider the process centering and the process can be out of control. The fact that the X-bars are within limits does not speak to individual x's or to the overall spread of the subgroups. I suspect, as you do, that the range chart, or sigma chart as the case may be, is not in control.

raghuraman,

With regard to CPK of 1.33, that is an arbitrary value that basically allows the process some wiggle room with in the specification limits while still being in control. As the CPK goes up the wiggle room gets greater. Keep in mind that process variation and process centering are two separate process characteristics that are conveniently measured together by Cpk. They should be understood separately from each other. Remember, the process does not know what the specifications are. The process can be statistical control and capable or not capable depending on how we arbritrarily set the specification.

Rick

Also see: Process Capability - Cp vs. Cpk for a good visual showing the distinction.