Revision Date: 2005-09-06
| In AIAG's Measurement Systems Analysis manual, the standard approach for R&R percentages is to use percent of study variation. This means that the parts selected for the study are used to estimate the product's variation (more correctly the product's process variation). This makes the selection of those parts critical to the analysis. You want to know if your measurement system can detect changes in the process and tell if the process goes out of control.
The prior methodology asked a different question: Can I tell whether a product is good or bad. This is an inspection mentality that the automotive manufacturers want to move away from. In this case, the percentages are based on dividing by the specification range.
My general suggestion is to use the percent of study variation. However, given the push to improve Cp and Cpk, we have reduced the variation in our processes considerably. In some cases, we have Cpk's > 2. In most of these cases, the R&R percentage will be poor because of the reduced variation in the processes. We can improve our measurement system (many times at great cost and sometimes we cannot improve it), but it will not result in a better product. So a suggestion that I frequently make is to specify some Cpk level above which you use R&R percent of tolerance and below which you use R&R percent of study.
Gordon Constable, Ph.D.
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