Measuring Your Capability: Cpk, Ppk, Cp and Pp
As we keep walking down this wonderful world of 6Sigma it’s important that we talk about how capability is measured. We’ve been talking about process capability for a few weeks now, so let’s talk about the capability measurement methods. This week we are going to focus on capability index and process capability.
What does it mean?
The first thing we need to understand are the terms for measurement, so here are a few basic definitions.
Cpk and Cp are capability rates and Pp and PPk are performance rates.
Cp- When you see this, you’re talking about rate of your process capability. To find it you use this formula:
Pp-When this comes up, the conversation is speaking to the pure performance of your process. The formula to find this data is:
Cpk- This refers to your process capability index, basically telling you how close your project is running to the acceptable limits. The formula for finding Cpk is:
Ppk-This refers to the non-centered distribution, when you hear this term it’s referring to adjustments to the effects that distribution. The formula for Ppk is:
What’s the Difference?
The main difference is the way the information is calculated. Cp and Pp is really short term data that considers only the quantity of information determined by specified limits. Cpk and Ppk rates process capability based on centralization and variation within one specification limit.
Data is so much more than numbers, but by understanding the why and the how 6Sigma begins to teach us what is significant in our data.