15. MSA – Type III Gage R&R
MSA’s type III study is applied when measurement system will be influenced by equipment variance mainly. It also will consider the part to part variance within the system. In general, the minimum sample size shall be at least 30 (10 non-destructive different samples measured for at least three times by same operator and same equipment) to have close to the normal distribution.
For the example demonstrated below, same operators measured 25 different samples for at least 3 times. Therefore 75 data points (25 x 3) were collected to analyze measurement system’s gage R&R, and the results running from Minitab can provide us the following explanation.
First, let’s start with how to interpret both the X-Bar and R chart from the analysis.
- From the X-Bar chart, only has 4 points fell out control limit, this indicates the part to part variance is not significantly different compare to gage variance.
- From the R-chart, only has 4 points fell out control limit, this indicates the part measurement were generally uniformed since only one particular part’s range falls outside of the calculated control limit.
X Bar and R Charts for the instrument/gauge.
If measurement system is used for process improvement, use %StudyVar for better estimation for measurement accuracy.
If measurement system to used for evaluating the parts’ relation with respect to specification, then %Tolerance is a more appropriate indicator.
Based on the analysis result on right hand side, both measurement repeatability and reproducibility are less than 20% (total gage R&R contributes from 13.41% of Repeatability based on the tolerance), this indicates the consistency of measurement system is in acceptable condition and can be improved due to the high variance contribution without considering the specification tolerance.
The measurement system’s Number of distinct category indicates it can be detect 1 group (ndc = 1), but this is reasonable because this gage is used for evaluating one particular part.
Minitab Summaries for Equipment Gage R&R