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Precision and Accuracy
Precision and accuracy are two components of the measurement system error. These components are often linked because their similar root causes for the error, although the error manifests itself differently.
When to use
Whenever a measurement system is being used, it should be checked for precision and accuracy. Depending upon the characteristics of the system, this check should be done periodically.
Precision and accuracy are two of the components of measurement system error. They have similar causes, but the impact they have on the measurement is different.
Accuracy errors cause a shift in the measured result. On average, the results are higher or lower than the true value. The amount of this shift is called bias. This shift is often addressed through calibration, which attempts to adjust the measurement system so as to remove bias.
Precision error is the common cause error of the system and reflects the normal variation in the results. Precision error is usually categorized as repeatability error and reproducibility error.
- Repeatability is often referred to as equipment variation. It addresses the question, “Can the same person, using the same measurement system, in the same way to measure the same item get the same answer?”
- Reproducibility is often referred to as operator variation. It addresses the question, “Can a different person, using the same measurement system, in the same way to measure the same item get the same answer?”
Both of these precision error categories are evaluated using a Gage Repeatability and Reproducibility (R&R) Study. The impact of accuracy errors is to shift the average value of the measurement by the amount of the bias. The impact of the precision errors is to increase the variation from measurement to measurement. Although the impacts are different, they have similar causes.
- Operator impact – in this case the error is due to changing operators. One operator’s technique may create a bias in the measurement that does not exist with a different technique (accuracy). Also, an operator’s training or skills may lead to different levels of variation and uncertainty in the use of the equipment (precision).
- Equipment impact – some equipment may have a built-in bias due to system fabrication, assembly or calibration that is not found in other equipment (accuracy). Also, the complexity of measurement equipment can lead to difficulties in setup or operation that create significant levels of uncertainty and variation in the measurement. This can be further compounded by variation in the system components due to wear and tear creating variation (precision).
- Other environmental factors – these can impact both operators and equipment. Environmental factors such as temperature may affect the operation of components and create a bias in equipment (accuracy). Or poor lighting may make it difficult for an operator to correctly read a dial gauge (precision).
Hints & tips
- It is important to understand the nature of the measurement error – accuracy or precision. Chasing the variation of a poor precision like it is an accuracy problem is likely to lead to even greater errors due to over-control. A Gage R&R study will show you the level of precision errors. First attain precision, then with calibration attain accuracy.
- Improving the impacts on measurement systems will often improve both accuracy and precision.
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