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Measurement Systems Analysis

Measurements

Chapter Four 

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Measurement Systems Analysis 

Measurement accuracy andprecision

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Measurement Systems Analysis 

Measurement SystemsAnalysis

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gage product observed        

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Measurement Systems Analysis 

Measured Value

Measured Value = ƒ(TV + Ac + Rep + Rpr) 

TV = true value

 Ac = gauge accuracyRep = gauge repeatabilityRpr = gauge reproducibility 

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Measurement Systems Analysis 

Measurement systemcomponents 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

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Measurement error 

Measurement error is considered to be

the difference between a value measured

and the true value.

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Types of gage variation

Systematic variation

•  Accuracy - improper calibration

•

Reproducibility - different persons usingsame equipment with different techniques

Periodic variation

•

Stability - wear, deterioration, environment Random variation

• Repeatability (unable to locate part to be

measured)

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Types of measurementvariation Accuracy

Stability

Reproducibility

Repeatability

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AccuracyDifference between the true average and theobserved average.

(True average may be obtained by using a more precisemeasuring tool)

True average

Observedaverage

Accuracy ormean bias

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Stability

Time 1 Time 2

The difference in the average of at least 2 sets ofmeasurements obtained with a gage over time.

Stability

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Reproducibility

Operator B

Operator A

Operator C

True Average

Variation in average of measurements made bydifferent operators using the same gage measuringthe same part.

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How do we improve gagecapability? Reproducibility

• operator training, or 

•

more clearly define measurement scaleavailable to the operator 

Repeatability

•

gage maintenance• gage redesign to better fit application