Gage R&R (Gage Repeatability and Reproducibility):-
Measurement System Analysis (MSA) is an important part of Measure Phase in DMAIC. Gage R&R is the type of Measurement System Analysis (MSA) that is used to quantify how well a variable (continuous) measurement system is working
We must confirm Resolution, Accuracy, Precision and stability of measuring devises before use them in measurement for generating data.
Resolution (or Discrimination) is the smallest division on the scale of the gauge or the smallest difference in reading that can be shown on the display of a measuring instrument. The higher the resolution, the smaller the measurement it can record
Basically, we can classify the error of measurement system in three main categories
1. Accuracy, 2. Precision and 3. Stability
Resolution – help use in selection the gage as per our process requirements (Part size , specification, standard deviation)
1. Accuracy: – Accuracy is difference between measured value and actual value of Part that measured. Main component of accuracy measurement is
a. Bias: It is difference between true value and measured value of part. some time we do not have actual true value of part then we can take the average of several measurements with the most accurate measuring equipment.
b. Linearity is the consistency of the gauge across its entire range. It is about how part size affects in the bias of measurement system. We should calculate the bias value of all expected range of part size according over process. some time it is proportional or inverse proportional to part size.
2. Precision: -It is the variation observed between repeated observations of the same unit – either due to operator (Reproducibility) or due to equipment (Repeatability). It is about the variation in the measurement. When we measure the same part many times with same devices then the below listed variation occur.
c. Repeatability: When same operator measures the same part many times with same measuring device then it is called the repeatability. It is a variation due to measuring device.
d. Reproducibility: – It is about the variation between the operators. If same part measures by more then one operator with same measuring device called reproducibility.
3.Stability: – Stability is about how accuracy and precision of the system perform over a time. It is consistency in the gauge over a period of time. Stability is observed if observed values are same when the same operator measures the same unit with the same measuring equipment over an extended period of time
Right order of 5 things during performing Gage R&R as below
Gage R&R tool measures the amount of variation in measurement system. Variation could be from the device or from the people.
“If you cannot measure it, you cannot improve it” – Kelvin
In the below diagram,
Ref 1.1 is a classical example for High Precision and Low Accuracy. Even though Precision is high, values/points are highly biased and inaccurate.
In Ref 1.3, values/points are both Accurate and Precise.
Resolution is pivotal in measurement systems as it discriminates between measurement value. Post looking
Total variation = Part variation + Measurement variation (error)