# What is Cgk? (Index of Testing Means Capability)

A

#### Atul Khandekar

Can anyone point to any references about what exactly is Cgk (Index of Testing Means Capability) and how is it calculated?

I found only one reference to it on the web at http://www.csigmbh.de (Sorry, I don't remember the complete URL of the page right now)

Atul Khandekar

B

#### Brian Dowsett

Atul,
In a previous employment we used to calculate Cg which was done by comparing six standard deviations (from the measurement system) to one tenth of the process specification limits.
(tolerance/10)/(6*SD of measurement system).
I guess if you used a traceable master to do your study, you could then apply +/- one fifth of the spec to produce an upper and lower limit. Then calculate Cgk upper & lower using 3 SD's as in Cpk calcs.
-for instance Cgk upper = "Upper Limit"-trial mean/ (3* trial SD)

Seems like a lot of effort to produce a capability index that no-one recognises though!
Cheers

Brian
I think the old (outdated) Ford R&R manual had details on this.

#### Marc

##### Fully vaccinated are you?
From: Tomo Doran
Newsgroups: misc.industry.quality
Subject: Re: What is Cgk?
Date: 29 Mar 2000 03:51:31 GMT

The capability indices Cg & Cgk are related to gaging and compare the variation and location to a reference figure - often 10% of process tolerance in the same way that Cp & Cpk is derived.

#### Marc

##### Fully vaccinated are you?
Does anyone have any contemporary information / comments on Cgk (Index of Testing Means Capability)?

#### Miner

##### Forum Moderator
There is not much information available on this topic, which means that it never really took off.

The short story is:

Cg = 0.2 * StDev(Process) / StDev(Gage)

and

Cgk = [3 * StDev(Process) - |Bias|] / [3 * StDev(Gage)]

Cg is really rendundant if you have %GRR. It adds no new information, just packages it differently. Cgk is only of use if you did not establish credible calibration acceptance limits. If that was done, Cgk adds little value.

#### Ajit Basrur

Ah..that was my first post at the Cove!
Miner is right. It is one way of calculating measurement capability index.

Oh, you still remember it

S

#### stefanhg

Cg/Cgk are Instrument Capability Indexes.
The idea for Cg is the same than for Cp, the "conceptual" formula for Cg is:
Cg = (allowable variation)/(actual variation

The differences are what we take for allowable and actual variation. For Cp, the actual variation is the process variation, and the allowed variation is the specification range.
For Cg, the actual variation is the INSTRUMENT ALONE variation (6 x Sinst No between parts, within part, between operators or along time variation is included. About 50 measurements are made consecutively on a master gauge or "best available" part, and always on the same point of the part, and by the same person, and in a controlled environment (usually the metrology room
The allowable variation is taken as 1/5 of the process variation (6 x Sproc) or 1/5 of the product specification range, depending on whether you want the instrument to control the process or to check conformance of the parts, then:
Cg=(0.2 x 6 x Sproc)/(6 x Sinst)=Sproc/5Sinst
or
Cg=(0.2 x Tol))/(6 x Sinst)=Tol/30Sinst
Cgk, as Cpk, takes into account the position (which would be the bias):

Cgk=(0.5 of the allowable variation - |bias|) / (0.5 of the actual variation)

The idea of Cg/Cgk is to have this information in advance to use it for the decision about if the instrument would be "selectable" for a given measurement.

T

#### Tcchen

Hello everyone,
Does anyone have any reference about Cgk? I know how to calculate Cg, Cgk, and their meanings, but just can't find any reference about those indicators. It's unlike GRR which we can find in AIAG MSA manual. I will be appreciate if someone can provide me any information. Thanks a lot.