# bias linearity in MSA

Six Sigma – iSixSigma Forums Old Forums General bias linearity in MSA

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• #67667

Ken K.
Participant

Because the whole idea of MSA is to understand the sources of variation that exist in your measurement system. If the accuracy of the measuring device varies significantly over the operating range (range of measurement), then don’t you think that is something you’d want to know?

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#67669

Chaz Weyer
Participant

If i understand your question correctly you would want to know about importance of bias and linearity in MSA. First definitions; bias is a measure of diffrence between measured values (on a given measurment equipment) and true (referenced to some traceable standard) value. Linearity is a measure of bias deviation accross measurment range. I will give an example : we had a few optical detectors that measured diffrent absolute values even though the MSA studies showed that they were very much reapeatable (7%). The problem was that at low optical power the detector response (input power versus readings) the noise/scattered light was high enough to move the curve out of linear relationship – I was seeing more light that I should.
Needless to say that none of the bias or linearity can be detected in MSA.
Chaz

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#67671

Neil Polhemus
Participant

What we are concerned about with gage linearity is that the measurements from a gage should increase linearly with the response or, if a gage is biased, the bias should remain constant over the range of measurements for which that gage is used. While we can tolerate a little non-linearity, it should be small with respect to the normal process variation. As typically defined, the percent linearity of a gage is related to the slope of a regression line fitting bias versus reference value. In general, the smaller the percent linearity the better.

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#27557

alita
Participant

Hi friends!
Please tell me why linearity of bias is important in MSA?

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