Mayberry, Ppk: Episode 1

Making intuitive sense of capability output

Mon, 08/27/2012 - 11:32

In this episode, Sheriff Andy Taylor uses cars and parking spaces to explain process capability to Deputy Barney Fife.

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Sheriff Taylor: What's wrong, Barney?
Barney: Someone spilled alphabet soup all over my capability output! Wait till I catch the practical joker who did this! They'll be sorry!!
Sheriff Taylor: That's not alphabet soup, Barney. Those are capability indices.
Barney: I don't see any indecisives, Andy. I just see a big mess of letters: Cp, Pp, Cpk, Ppk, Cpm, PPM... LMAO!
Sheriff Taylor: LMAO is not part of capability output, Barney. At any rate, the indices help you evaluate whether your process is meeting your customer requirements.

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Comments

Nice article, Patrick. I have been using the car analogy for several years to help people understand capability. My analogy is slightly different in that I liken the width of the car as the process variation and the width of a garage door as the specification width. You need a Pp of at least 1.0 to have any chance of being able to get the car into the garage without scraping the paint or damaging the garage, but only if you have centered the car. If you have had a few drinks on the way home, you need a Pp larger than 1.0 to avoid the damage (allowing for some drift)! Then, of course, you have to consider the Ppk and/or Cpk in order to understand that you actually got the car into the garage safely. I'm sure these will be discussed in subsequent episodes. Best regards, Steve

Thanks for your nice comment, Steve. I like your take on the car parking analogy—especially the drift and the garage door. Reminds me of when I first got my driver’s license at age 16 and ripped out half the garage door frame while backing out for the first time.  Good thing process variation is not 3-dimensional, or parking your process between the spec limits would be even more fraught with peril! (You’re right about needing to consider Ppk/Cpk along with Pp/Cp. I was nervous about breaking the post into 2 separate episodes for that very reason—but ultimately decided to present the info in bite-sized, easily digestible chunks. So Barney gets the Cpk/Ppk story in Episode 2). Cheers, Patrick

Thanks for your feedback, Steve.  I was debating adding episode 3 to cover Cpm/Ppm! But I received mixed feedback on how often practictioners perform capability analysis with a target value. One quality analyst told me it's not very common to use a target in the context of evaluating process capability (although there is an option in Minitab to enter a target to obtain the Ppm/Cpm estimates if one desires that output). I'm curious of your (and others') reaction to this feedback--would you disagree with the idea that it's not common to enter a target when performing a capability analysis for most applications?

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