# Practical Quality Engineering Resources Pk Ppk Short and Long Term Capability 2016-02-22

Definitions of long and short term capablity

1. ### Bev DModeratorStaff Member

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2. ### MinerModeratorStaff Member

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3. ### ncwalkerWell-Known Member

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In Wherler's article, both Ppk and Cpk have 6s in the denominator. Shouldn't that be 3s?

4. ### MinerModeratorStaff Member

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Notice that he added a 2 in the numerator.

5. ### ncwalkerWell-Known Member

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I did. And now that you say it, it makes sense. But when I originally looked at it I took his meaning of "2 DNS" to be check BOTH "Distance to Nearest Specification". Not 2 times the Distance to Nearest Specification (DNS). Which would be redundant because "Distance to Nearest Specification" implies you would be checking both sides.

So yes. Math works out in my head now. But it could have been more clear. Like maybe a multiplication symbol ....

6. ### MinerModeratorStaff Member

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It depends. Which is clearer?
a) Y = 2 + 3X - 5X^2
b) Y = 2 + 3*X - 5*X^2

Sometimes the multiplication symbol clarifies, sometimes it muddies the waters.

7. ### ncwalkerWell-Known Member

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Well, if you want to argue, ;-)

YOU'RE example is clear. Because it is "3X" and "5X". What Wheeler had would have been "3 X" and "5 X". Which looks like something is missing OR it is enumeration.

8. ### ncwalkerWell-Known Member

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And I would also posit, it never muddies the waters. It may visually appear messier, but that's not the same as clarity of an equation. It always makes it more clear when you don't take the accepted shortcuts in math.

9. ### ncwalkerWell-Known Member

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