Improving Measurement Systems

The key to improving the measurement system is to get to the root cause of the problem.

• The results of the GR&R study can help here.

• Use the relative magnitude of both the Repeatability and Reproducibility to determine where to start the investigation.

If the Repeatability calculation is greater than the Reproducibility:

• Begin by looking at the measurement equipment since repeatability deals with variation due to the test equipment.

• Here are some possible causes of a high repeatability calculation:

  • Measurement device may need to be repaired.
  • The device or fixture is not rigid enough for the measurement being made.
  • Measurement discrimination cannot meet the level of precision necessary for measuring the part.
  • Excessive within-part variation can also appear as a measurement repeatability problem (for example a round part that is not concentric).
  • Lighting may make it difficult to accurately read the scale of the instrument.

If the Reproducibility calculation is greater than the Repeatability:

• First look at the appraisers and their test methods.

• Here are some possible causes of a high reproducibility rating:

  • Is one appraiser’s overall average part measurement significantly different than the others? Retraining is needed.
  • Appraisers may have a hard time reading the gage or device.
  • The part or sample could be too difficult to handle or too awkward to align while taking a measurement.