Measurement error is the difference between the actual value of a characteristic being measured and the value obtained by the measurement system. Given that the measurement device is discriminate (sensitive to variation of the product being measured), with each repeat of a measurement, there will be variation.
By the way, if no variation is seen, measure with a more discriminate device. There is systemic error and random error. The systemic error occurs when we use the measurement device in the same way and in the same condition. The random error will vary from reading to reading. There can also be variation due to the human element (when applicable).
Use: There really is no use for measurement error, but we need to be aware that it exists and we need to continually reduce it as much as possible. As we improve our processes through the use of Six Sigma, we may need to invest in a measurement system that is more discriminate (finer) to detect smaller variations in the product. The measurement error is also broken into two groupings – – error due to reproducibility and error due to repeatability.
If we were to take five individuals and have them measure the same part, in the same place, five times, using just one measurement device, any variation in the readings would be due to measurement error. Some of that measurement error might be due to person-to-person variation. Some of the measurement error might be due to the measurement device. And even if the study used only one measurement device, some of the error would be due to within-device variation. There are two primary ways of evaluating the measurement system.
Some practitioners use a gage R&R study which is useful in giving you percentages of the error that are due to repeatability and reproducibility. The shortcoming of the gage R&R system is that it doesn’t give you a picture of the variation. Others use the Dr. Wheeler method that uses control charts to evaluate measurement systems. This author favors the Wheeler method because seeing the enemy (variation), makes the variation visible.
