# ?Pa? Capability Index - Has anyone ever heard of this?

#### debun

##### Starting to get Involved
I got a spread sheet from a vendor where they calculated a capability in excel called ?Pa? I know of Pp and Cp but I have never heard of Pa. The excel formula calculated it using the absolute value of ? - center value = (USL+LSL)/2 divided by 3σ so in summary ?-center value / 3σ. Anyone ever heard of this?

#### Stijloor

Staff member
Super Moderator
Re: ?Pa? capability index, has anyone ever heard of this?

A Quick Bump!

Can someone help?

Thank you very much!

T

#### t.PoN

I always thought capability index is used only for normal distribution.
which means: Center Value = μ!

One possible interpretation is the Probability of Acceptance! but still the formula doesn't add up

Last edited by a moderator:

#### debun

##### Starting to get Involved
I always thought capability index is used only for normal distribution.
which means: Center Value = μ!

One possible interpretation is the Probability of Acceptance! but still the formula doesn't add up
I don't think I was very clear

center value = (USL+LSL)/2
μ = mean of the population or in this case sample mean

Pa = (?-center value )/3σ

#### Bev D

##### Heretical Statistician
Staff member
Super Moderator
I've not heard of it - but then I tend to disregard any literature on the capability indices.

The math is simple, but I'm not really sure of it's value: the formula just results in a ratio of the distance of the mean of the process from the center of the specification to the spread of the individual values. without a comparison to the specifications there is no way to know if the process is 'good' or 'bad'.

Did they provide you with any 'acceptable number?

Or is this just an interim calculation that is used in another formula?

#### debun

##### Starting to get Involved
I've not heard of it - but then I tend to disregard any literature on the capability indices.

The math is simple, but I'm not really sure of it's value: the formula just results in a ratio of the distance of the mean of the process from the center of the specification to the spread of the individual values. without a comparison to the specifications there is no way to know if the process is 'good' or 'bad'.

Did they provide you with any 'acceptable number?

Or is this just an interim calculation that is used in another formula?
They just have a cell called "Pa" with Chinese 準確度 which according to Google translate means accuracy. It is not used in any other calculations and no acceptance criteria for the value.

They have the same Chinese characters for the "Pp" cell, but of course the formula is different.

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