As with process performance, measurement system performance is the net effect of all significant and determinable sources of variation over time.
Performance quantifies the long-term assessment of combined measurement errors (random and systematic). Therefore, performance includes the long-term error components of:
Capability (short-term errors)
Stability and consistency.
An estimate of measurement performance is an expression of the expected error for defined conditions, scope and range of the measurement system (unlike measurement uncertainty, which is an expression of the expected range of error or values associated with a measurement result). The performance expression of combined variation (variance) when the measurement errors are uncorrelated (random and independent) can be quantified as:
Again, just as short-term capability, long-term performance is always associated with a defined scope of measurement – conditions, range and time. The scope for an estimate of measurement performance could be very specific or a general statement of operation, over a limited portion or entire measurement range. Long-term could mean: the average of several capability assessments over time, the long-term average error from a measurement control chart, an assessment of calibration records or multiple linearity studies, or average error from several GRR studies over the life and range of the measurement system. A statement of measurement performance need only be as complete as to reasonably represent the conditions and range of measurement.
Long-term consistency and uniformity (repeatability errors) over the range of measurement are included in a performance estimate. The measurement analyst must be aware of potential correlation of errors so as to not overestimate the performance estimate. This depends on how the component errors were determined. When long-term (uncorrected) linearity, uniformity or consistency vary significantly over the range, the measurement planner and analyst has only two practical choices:
1) Report the maximum (worst case) performance for the entire defined conditions, scope and range of the measurement system,
or
2) Determine and report multiple performance assessments for a defined portion of the measurement range (i.e., low, mid, larger range).
source of process performance: Analysis of measurement systems
