Gage R&R Bias and Linearity

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Gage R&R Bias and Linearity

Two other factors affect the accuracy of your measurement system: Bias and Linearity.

Bias: Does your gage tend to over read or under read the same size part?
(Imagine measuring the length or diameter of a steel rod with known dimensions.)

Linearity: Does your gage over read or under read across a range of different sized parts?
(Imagine using the gage on tin cans of various diameters, from small, 6 oz. juice cans to 64 oz., family-sized cans.)

If you want to know the "bias" of your gage, simply input the "target" or "reference" value for the parts being measured into a software package like the QI Macros Gage R&R template, and the template will calculate the bias of the gage (plus or minus).

If you want to know the "bias and linearity" of your gage, switch to the linearity worksheet in the QI Macros Gage R&R template and conduct a linearity study.

Linearity Study
To conduct a linearity study, you will need five parts of DIFFERENT sizes that have been accurately measured.

1. Have each of the five parts measured 12 times.
   
2. Input the data into the Gage R&R template's Linearity worksheet
   
3. Input the accurate measurements for each part as a reference
   
4. Analyze the linearity using the line graph on the worksheet.

Ideally, there shouldn't be any change in bias from small to large. If you look at the line graph, it should be a horizontal line. More often, however, a gage may over read the small and under read the large.
If there is too much slope to the line (too much bias), you may want to use the gage in its optimal range and find other gages to measure where this gage's bias is too large.

Conclusion
So, there, in a nutshell Bias and Linearity. Your goal is to minimize the amount of variation and error introduced by measurement, so that you can focus on part variation. This, of course, leads you back into the other root causes of variation: process, machines, and materials.

If you manufacture anything, measurement system analysis can help you improve the quality of your products, get more business from big customers, and baffle your competition. Enjoy.

AIAG Measurement System Analysis Gage R&R Overview

Gage R&R Destructive Testing

AIAG Case Study - Non-replicable

Gage R&R Software
Obviously, crunching all of the numbers for a Gage R&R study is non-trivial, but you can do it easily with the Gage R&R template for Excel. Buy one separately or as part of the QI Macros for Excel SPC Software.

Order Gage R&R Template

Gage RR Templates - $99 Download Only

Order the QI Macros for Excel SPC Software

QI Macros Excel SPC Software for Histograms and Control Charts $139 Plus S&H

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The QI Macros draws line, pie, bar, pareto, box whisker, histogram (Cp, Cpk), scatter and control charts (with stability analysis).

Templates include the Ishikawa diagram, QFD, DOE, FMEA, PPAP, and Gage R&R for MSA. Performs ANOVA, t-test, F-test, and regression analysis.

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