What is Evaluating the Measurement Process (EMP)?

caliperIt is time to replace the average/range method and ANOVA method for Gage R&R studies. Evaluating the Measurement Process (EMP) is a collection of techniques that allows you to discover much more about your measurement process than if you just use those traditional techniques. EMP has been developed over the years by Dr. Donald J. Wheeler.

The experimental process is essentially the same. One or more operators, measure one or more parts, multiple times. The difference is that EMP immediately places the results on a control chart to assess how consistent, reproducible, and repeatable the measurement system is before moving forward with the characterization (calculation) of the various statistics. 

Plus, EMP introduces an entirely new way of addressing “how good” your measurement is. It is not simply based on the % of the variance due to the measurement. The method EMP uses answers three questions:

  • How the measurement system can reduce the strength of a signal (out of control point) on a control chart.
  • The chance of the measurement system detecting a large shift.
  • The ability of the measurement system to track process improvements.

Dr. Wheeler’s book, EMP III: Evaluating the Measurement Process and Using Imperfect Data, introduces the three core techniques in EMP:

  • Consistency Study
  • Short EMP Study
  • Basic EMP Study

A Consistency Study is run by having one operator measure the same part multiple times. The results are then placed on a individuals (X-mR) control chart. The control chart must be in statistical control before proceeding. If it not, the measurement process cannot really measure anything until the reasons for the out of control points are found and corrected. Once the measurement process is consistent, you can move forward with the characterization of the measurement process (e.g., calculating the measurement error and bias).

A Short EMP Study is run by having one operator measure multiple parts multiple times. Using different parts allows you to begin to assess if the measurement process can tell the difference between parts. The results are first placed on a X-R control chart. Each subgroup is composed of the measurements on one part. The range is then an estimate of the measurement error (like in the Consistency Study). The range chart must be in statistical control to move forward with the characterization of the measurement process. The average range is used in the calculation of the control limits for the X chart. Since the average range represents just measurement error, you want the part averages on the X chart to be out of control. This means that the measurement process can tell the difference between parts in the process. If the parts are representative of the process, you can use that variation to determine the total variance and part variance. If the parts are not representative, you can always use a historical value for the process sigma.

The Basic EMP Study is performed with multiple operators running multiple parts multiple times. The results are assessed first by placing them on X-R control chart using the results for one operator and one part to form the subgroups. The R control chart must be in control before you can characterize the measurement process. Again, the range chart represents measurement error. The analysis technique also includes using the Analysis of Main Effects to look for differences between the operator averages and the Analysis of Mean Ranges to look for differences in the measurement error between operators.

Once you have learned EMP, you really have to question why you would use the older Average/Range and ANOVA methods. There is so much more you learn from the EMP studies. In addition to the above, the analysis technique answers these questions for you:

  • Do I have enough data in the study?
  • Is the measurement process resolution effective?
  • How good is my measurement process (Class 1, Class 2, Class 3, Class 4)?
  • What should my manufacturing specifications be?
  • What is the true precision to tolerance ratio?

For more information on the EMP techniques, please visit our SPC Knowledge Base

Also, our SPC for Excel software includes the Consistency Study, the Short EMP Study, and the Basic EMP Study.

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