1. Measurement Systems Analysis
Chapter Four
Measurement Systems Analysis

2. Measurement accuracy and precision
Measurement Systems Analysis

3. Accuracy and precision
Measurements are said to be accurate if their tendency is to center around the actual value of the entity being measured.
Measurements are precise if they differ from one another by a small amount.
What can you say about the causes relating to each type?
Measurement Systems Analysis

4. Measurement Systems Analysis

5. Measurement Systems Analysis
Measured Value
Measured Value = ƒ(TV + Ac + Rep + Rpr)
TV = true value
Ac = gauge accuracy
Rep = gauge repeatability
Rpr = gauge reproducibility
Measurement Systems Analysis

6. Measurement system components
Equipment or gage
Type of gage
Attribute: go-no go, vision systems (part present or not present)
Variable: calipers, probe, coordinate measurement machines
Unit of measurement - usually at least 1/10 of tolerance
Operator and operating instructions
Measurement Systems Analysis

7. Measurement Systems Analysis
Measurement error
Measurement error is considered to be the difference between a value measured and the true value.
Measurement Systems Analysis

8. Types of gage variation
Systematic variation
Accuracy - improper calibration
Reproducibility - different persons using same equipment with different techniques
Periodic variation
Stability - wear, deterioration, environment
Random variation
Repeatability (unable to locate part to be measured)
Measurement Systems Analysis

9. Types of measurement variation
Accuracy
Stability
Reproducibility
Repeatability
Measurement Systems Analysis

10. Measurement Systems Analysis
Accuracy
Difference between the true average and the
observed average.
(True average may be obtained by using a more precise measuring tool)
Accuracy or
mean bias
True average
Observed
average
Measurement Systems Analysis

11. Measurement Systems Analysis
Stability
The difference in the average of at least 2 sets of measurements obtained with a gage over time.
Stability
Time 1
Time 2
Measurement Systems Analysis

12. Measurement Systems Analysis
Reproducibility
Variation in average of measurements made by different operators using the same gage measuring the same part.
True Average
Operator C
Operator B
Operator A
Measurement Systems Analysis

13. Measurement Systems Analysis
Repeatability
The random variation in measurements when one operator uses the same gage to measure the same part several times.
Observed Average
True Average
Repeatability
Measurement Systems Analysis

14. How do we improve gage capability?
Reproducibility
operator training, or
more clearly define measurement scale available to the operator
Repeatability
gage maintenance
gage redesign to better fit application
Measurement Systems Analysis