What are the sources of process variation?

Observed process variation is broken into two sources:
Part-to-part variation
The variability in measurements across different parts. Ideally, differences between parts (Part-to-Part) should explain most of the variability.
Measurement system variation
Measurement system variation is all variation associated with a measurement process. Potential sources of variation include gages, standards, procedures, software, environmental components, and so on.
Like any other process, a measurement system is subject to both common-cause and special-cause variation. To control the measurement system variation, you must first identify the sources of the variation, then you must either eliminate or reduce the various causes. Measurement system variation is split into two sources:
Repeatability
Repeatability is the variation due to the measurement device. It is the variation that is observed when the same operator measures the same part many times, using the same gage, under the same conditions.
Reproducibility
Reproducibility is the variation due to the measurement system. It is the variation that is observed when different operators measure the same part many times, using the same gage, under the same conditions.
Reproducibility can be split into two sources:
  • Operator: The variability in measurements due to different operators.
  • Operator-by-Part: The variability in measurements due to different operator/part combinations after considering part and operator separately.
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