The Effects of Measurement System Variability

Because the measurement system can be affected by various sources of variation, repeated readings on the same part do not yield the same, identical result. Readings vary from each other due to common and special causes.(System Variability)
The effects of the various sources of variation on the measurement system should be evaluated over a short and long period of time. The measurement system capability is the measurement system (random) error over a short period of time. It is the combination of errors quantified by linearity, uniformity, repeatability and reproducibility. The measurement system performance, as with process performance, is the effect of all sources of variation over time. This is accomplished by determining whether our process is in statistical control (i.e., stable and consistent; variation is due only to common causes), on target (no bias), and has acceptable variation (gage repeatability and reproducibility (GRR)) over the range of expected results. This adds stability and consistency to the measurement system capability.

Because the output of the measurement system is used in making a decision about the product and the process, the cumulative effect of all the sources of variation is often called measurement system error, or sometimes just “error.”

source of System Variability : Analysis of measurement systems