MSA Example: Attribute or

Categorical Data

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MSA Operational Definitions

Accuracy: Overall agreement of the measured value with the true value (which may be an “expert” value). Bias plus precision.

Attribute Data: Discrete qualitative data.

Attribute Measurement System: Compares parts to a specific set of criteria and accepts the item if the criteria are satisfied.

Bias: A systematic difference from the true value. Revealed in the differences in averages from the true value.

Precision: Variation in the measurement process.

R&R: Repeatability and Reproducibility. Two elements of precision.

Repeatability: The variation observed when the same operator measures the same item repeatedly with the same device.

Reproducibility: The variation observed when different operators measure the same parts using the same device, sometimes it can be the same operator using different devices.

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“Is the variation (spread) of the measurement system too large to study the current level of process

variation?”

+ =

(Observed Variability)

Total VariabilityProduct VariabilityProcess Variability

Variationin the

measurementprocess

The Fundamental MSA Question

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TrueValue or Standard

Bias

Observed Average

Possible Causes of Bias

Sensor not properly calibrated

Improper use of sensor

Unclear procedures

Human limitations

Bias

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Repeatability

Possible Causes of Poor Repeatability

Equipment Gage instrument needs

maintenance The gage needs to be more

rigid

People Environmental conditions

(lighting, noise) Physical conditions (eyesight)

Repeatability

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Reproducibility

Mean of the measurements

of Operator BMean of

the measurementsof Operator A

Possible Causes of Poor Reproducibility

Measurement procedure is not clear

Operator is not properly trained in using and reading gage

Operational Definitions not established

Reproducibility

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Discrete qualitative data

Go/no-go basis; or limited data categories

Compares parts to specific criteria for accept/not accept or to be placed in category

Must screen for effectiveness to discern good parts from bad

At least two appraisers and two trials each

If available, have Quality Master rate parts first

Attribute Measurement Systems Study

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Attribute MSA Study

Attribute MSA ExampleAppraiser A Appraiser B

Master 1 2 1 21 G G G G G2 G G G G G3 G NG G G G4 NG NG NG NG NG5 G G G G G6 G G G NG G7 NG NG NG G NG8 NG NG NG G G9 G G G G G10 G G G G G11 G G G G G12 G G G G G13 G G NG G G14 G G G G G15 NG G G G G16 G G G G G17 G G G G G18 G G G G G19 G G G G G20 NG G G G G

G = GoodNG = Not Good

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Challenges of Continuous Process MSA

MSA study is an experiment

Requires two or more trials for calculating Repeatability

Needs a way to present the inspection units to the appraiser multiple times

Is not possible within the continuous process

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Case Example: Visual Inspection of Glass

Catwalk

Glass

Inspector

Cutter

Packers

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Case Example: Challenges to Overcome

Bias to the standard could be evaluated on-line.

Repeatability and Reproducibility (R & R) could not be evaluated on-line.

A method had to be devised to allow the inspectors to view the same pieces of glass repeatedly.

The solution was an off-line conveyor which simulated the on-line condition as closely as possible.

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20 pieces of glass from the process that included both good and bad samples were selected.

A team of people well versed in the quality standard classified each piece of glass as either “pass” or “fail.”

All regular inspectors independently evaluated each piece twice (in random order).

The inspectors used a log sheet to record the data.

Minitab® was used to analyze the data.

Case Example: Attribute MSA Method Employed

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Excerpt of full data

for 20 inspectors

Case Example: Attribute MSA Study Data

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LarryClin

t

Ronn

ieJ o

hn

J ean

nie

Stev

eEa

rlTim

Moll

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100

90

80

70

60

50

40

Appraiser

Perc

ent

95.0% CIPercent

LarryClin

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Ronn

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J ean

nie

Stev

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rlTim

Moll

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100

90

80

70

60

50

40

Appraiser

Perc

ent

95.0% CIPercent

Date of study: 9/13 and 9/19/07Reported by: Bill Sunderland and Bob PanatieriName of product: SolarMisc: Mainline Marking

Assessment Agreement

Within Appraisers Appraiser vs Standard

Case Example: Attribute MSA Study Results

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Case Example: Attribute MSA Study Results (continued)

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Case Example: Attribute MSA Study Results (continued)

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Case Example: Attribute MSA Study Results (continued)

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What could have caused the poor agreement?

What was done to improve consistency?

Case Example: Attribute MSA Study Conclusions