Mayberry, Ppk: Making Intuitive Sense of Capability Output (Episode 1)
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.
Barney: Well, my customers don’t require random letters of the alphabet. I wish they did. It’d be a lot easier to satisfy ‘em!
Sheriff Taylor: Okay, let’s try to break this down for you, Barney—should we start with Pp?
Barney: No thanks, I just went a couple minutes ago…HA HA!
Sheriff Taylor (sighing): Hmm…this might be harder than I thought. Okay, let’s try an analogy. Think of the variation of your process as a car.
Barney: What kind of car?
Sheriff Taylor: Depends. If your process has a lot of variation, you might think of it as a long, stretched out limousine, like this:
Barney:. Oh boy, that’s what I call a lottttt-a-car!
Sheriff Taylor: Well, Barney, it’s a lot of process variation. But suppose your process doesn’t produce much variation. In that case it might be more like a small sports car, like this:
Barney: Boy, would I love to drive a zippy little process like that!
Sheriff Taylor: Wouldn’t we all. Now, you need a parking space to park your car in, don’t you Barney?
Barney: You're darn tootin' I do! I’d like a space right on Main Street Mayberry!!
Sherriff Taylor: Well, your parking space is determined by your customer requirements—in other words, the specification limits for your process. Those limits are the parking space that your car—your process—needs to fit into:
Barney: Well duh, I get that. You ain't talkin' to a rock, ya know! But what does that have to do with all those darn p’s in my output?
Andy: Well, Barney, Pp, for example, is just the ratio of the parking space to the length of the car.
Barney (excited): I get it! So Pp just tells me how many cars I can get into my parking space!
Sheriff Taylor: You got it, Barney! It’s how many times your process spread fits into your specification spread. So the smaller the car, or the larger the parking space, in relation to each other, the higher the value of Pp and the more capable the process is.
Barney: Eureka! So all those p’s stands for parking!!
Sheriff Taylor: Well…I guess you could say that, Barney...but don’t forget about the c’s in the output too, like Cp.
Barney: Just when I was getting a handle on things, you got to spring something new on me!
Sheriff Taylor: Don’t worry Barney. If you understand Pp, you understand Cp. They’re the same ratio—the spec spread to the process spread. The only difference is that Cp is calculated using the short-term variation of the process—the variation within subgroups—while Pp is calculated using overall variation of the process. So Cp tells you the short-term (potential) capability and Pp tells you the long-term, actual capability.
Barney (bored, playing with gun): Are ya done with your anamoly?
Sherriff Taylor: “Analogy,” Barney. Yes, I think that’s probably enough for one day. Why do you ask?
Barney: Because I got real work to do!
Sheriff Taylor: Real work?
Barney: Writing parking tickets, Andy! While we’ve been yapping, every Tom, Dick, and Harriet in Mayberry's been out there parking illegally on Main Street!!
Stay tuned for the next episode, when Barney learns how to use Cpk and Ppk to ticket processes that fall outside of their legal limits.