The measurement system is essential when you’re measuring because the wrong measurement system means inaccurate data.

This page is structured in this way:

  • Measurement Variation
    • Accuracy
      • Bias
      • Linearity
      • Stability
    • Precision
      • Repetability
      • Reproducibility

In a measurement system, one thing to know is the difference between accuracy and precision. 

  • Accuracy is how far the measure is from the actual value.
  • Precision is when I measure how far they are from each other.

So I can have a very precise system because the measurements are very near, but with low accuracy because they are very far from reality.

About Accuracy we need to know that:

  • The bias is the difference between the observed average measure to the true value.
  • Linearity is another definition related to bias and says how the measurement system works across a range. So you have a linear bias across all the different values or not? Maybe people have a low bias when measuring meters, but when they pass to km or millimeters, they have a high bias that isn’t linear. 
  • Stability is the capacity of a measurement system to produce the same value over time when you measure the same thing.

About Precision we need to asses:

  • Repeatability: When the same person measures something, you want to get the same value. A way to make this happen can be, for example, the use of a master sample. In this way, you can look at the difference between what you’re measuring and the sample to assess the correct measurement.
  • Reproducibility: When different people get different measures. You can assess this problem with a Standard Operation Procedure (SOP).

Gage Repeatability and Reproducibility (Gage R&R) measures the amount of variability in measurements caused by the measurement system itself. 

There are three different kinds of Gage R&R Study that are:

  • Crossed: We have N operators and M parts. Each of these N operators measures all the M parts;
  • Nested: We have more operators and more parts to test. Each operator test a different subset of parts. This is used when the test destroys the part.
  • Expanded: when you need to test more than two factors (not only parts and operator but also others).

The Gage R&R can be performed with 3 methods:

  • Range method: Give an approximation of measurement variability but without Repeatability and Reproducibility measure;
  • Average Range method: This method quantifies the variability and gives Repeatability and Reproducibility measures. Only Crossed Gage R&R can be performed with this method. X Bar R Control Charts is a way to perform this method.
  • Analysis of variance method: It’s the most accurate method for Repeatability and Reproducibility measurement. It also quantifies the variability of the interaction between operators and parts. ANOVA is a way to perform this method.
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