There are two critical characteristics to examine in a gage system. The two Measurement Methods are Precision and Accuracy
Accuracy: Accuracy is an unbiased true value and is normally reported as the difference between the average of a number of measurements and the true value. Checking a micro meter with a gage block is an example of an accuracy check.
Precision: In gage terminology, “repeatability” is often substituted for precision.
Repeatability is the ability to repeat the same measurement by the same operator at or near the same time.
The calibration of measuring instruments is necessary to maintain accuracy, but does not necessarily increase precision. In order to improve the accuracy and precision of a measurement process, it must have a defined test method and must be statistically stable.
Measurement Methods – Gage Repeatability and Reproducibility
Measurement system errors can be due to:
Repeatability – Variation in measurement when a person measures the same unit repeatedly with the same measuring gage(or tool)
Reproducibility - Variation in measurement when two or more persons measure the same unit using the same measuring gage(or tool)
There are certain thumb rules for acceptable level of Measurement System Variation.
For continuous data, if the percentage tolerance is less than 8%, percentage contribution is less than 2% and number of distinct categories is greater than 10, then the Measurement System is Acceptable. Likewise, we would evaluate the risks of the measurement system if the percentage tolerance is between 8% and 30%, percentage contribution is between 2% and 7.7% and number of distinct categories is between 4 and 10.
Measurement System Analysis – Variable and Attribute MSA
The observed process variation can be due to:
1. Variation from the Measurement System (and/or)
2. Actual process variation
Variation from Measurement system is due to
1. Variation from appraiser (and/or)
2. Variation from gage
3. Variation from appraiser is the variation caused by the user not using the gage as it should be used
4. Variation from gage is the variation caused by the gage due to inappropriate functionality.
Variation from gage can be further bifurcated into:
1. Accuracy: The difference between the average of observed values and the standard.
2. Stability: Variation in measurement when a person measures the same unit repeatedly with the same measuring gage (or tool).
3. Reproducibility: Variation in measurement when two or more persons measure the same unit using the same measuring gage (or tool).
4. Repeatability: Variation in measurement when the same person measures the same unit using the same measuringage (or tool) over an extended period of time.
5. Linearity: The consistency of the measurement across the entire range of the measuring gage.