Mayberry, Ppk: Making Intuitive Sense of Capability Output (Episode 2)
Sheriff Taylor: Remember the talk we had awhile back about Pp?
Barney: Keep talking like a potty mouth, Andy, and Aunt Bea’s going to wash your mouth out with soap!
Sherriff Taylor (sighs): So you remember that Pp is the ratio of the specification spread to—
Barney (interrupts): —the process spread! Yeah, yeah, I got all that. And the smaller your car (process spread), relative to its parking space (specification spread), the higher Pp will be—and the more capable your process. But it makes me wonder, Andy.
Sherriff Taylor: About what?
Barney: What’s the legal requirement for Pp? When should we start arresting quality engineers and throwing 'em in jail?
Sheriff Taylor: It really depends on the application, Barney, and the consequences of having parts fall outside of the specification limits. Some industries use a minimum requirement of 1.33 for capability indices. But if the consequences for customers are severe, 1.33 might not be high enough. If they're minor, a lower value might be okay.
Barney: I’ll use 1.33 as the legal minimum. Anything else is a violation in my book!
Sherriff Taylor: Even if your benchmark were acceptable to everyone, Barney, you can’t go arresting people based only on a Pp value.
Barney: And why not?
Sheriff Taylor: Because Pp doesn’t tell you the whole story.
Barney: Now you tell me! And just what’s the "whole story," may I ask?
Sheriff Taylor: Well, Pp just tells you whether your process spread fits into your specification limits. But it doesn’t tell you anything about the actual location of your process. For example, both of these processes have the same Pp.
Barney: So Pp tells you whether your car fits into the parking space. But it doesn’t tell you where your car is parked—whether it’s centered in that parking space or even inside the parking space.
Sherriff Taylor: Exactly! To assess the location of your process as well, you need another index—Ppk.
Sheriff Taylor: Ppk. It’s the ratio of the distance of the process center, the mean, to its closest specification limit, over half the process spread.
Barney: Can we talk about parking instead?
Sheriff Taylor: It’s the ratio of the distance from the center of the car, wherever it’s parked, to the parking boundary that’s closest, divided by half the length of the car:
Barney: So Ppk really just tells me how close my process is running to its nearest spec limit! But why does Ppk only consider the parking boundary that’s closest?
Sherriff Taylor: Well, it’s a worst case scenario, Barney. You're naturally more concerned with any potential violation of your specification limits.
Barney: Of course I am! And the legal regulations for Ppk?
Sheriff Taylor: Like Pp, it just depends. Higher is better of course. Anything less than 1 is not capable. 1.33 or above might cut it for many applications. Others require 1.66 or higher.
Barney: And now you’re going to tell me that Cpk is basically the same ratio as Ppk—just like Cp was basically the same ratio as Pp. Except Cp and Cpk just use the short-term process variation (within subgroups) instead of the long-term (overall) process variation.
Sheriff Taylor: Barney! I’m very impressed!
Barney: Of course you are! You can teach an old deputy new tricks, you know!
Sheriff Taylor: Looks like you've developed a good understanding of four main indices in your capability output: Cp, Cpk, Pp and Ppk.
Barney: You bet Aunt Bea's army boots I have!
Sherriff Taylor: Then you're ready to match the mug shots of four processes with their capability rap sheets. Without the cars, of course...
Barney (gulps): Without the…cars….?!
Help Barney Match these 4 Process Mugshots...
...with their Pp/Ppk Rap Sheets!
(To see answers, hold your cursor over a rap sheet)