# Gage R&R Studies - Goodness of fit, linearity and linearity percentage

L

#### lday38

linearity gage studies

Is there anyone who can help me in defining the acceptable factors when doing a linear gage study on goodness of fit, linearity and linearity percentage?I have the msa manual, but I am just not getting it.

#### Atul Khandekar

##### Quite Involved in Discussions
lday38,

I believe you are refering to the 2nd edition of MSA manual. There are no 'acceptable factors'. The concept of Linearity % has been dropped from the latest - the third edition. Here is an excerpt from AIAG's MSA FAQ page:
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Q.Why did we drop the %Bias and %Linearity?

The reason we dropped the indices is that (1) there is no "correct" way to analyze them and (2) we want to focus on the understanding of the measurement system variability and sources of variation rather than on "acceptable" indices.

We went with the focus that the bias and linearity should be the statistical equivalent of zero -- consequently the confidence bounds and the test of hypothesis. If the bounds are large (i.e. the natural measurement system variability is large) then the bias can be statistically zero even though it may not be "emotionally" zero (i.e. a large percent in the old terms). However, because the variability is large, the system is unacceptable due to the other parameter evaluations and, furthermore, adjusting the bias (using this variation) can cause the bias to become worse even though the calculated index becomes better -- ala Deming's funnel experiments.
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The third edition now incorporates some definitive criteria for acceptance of linearity (and bias) based on hypothesis tests.

- Atul.

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