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Measurement error. Roland Caulcutt Caulcutt Associates. Two processes. Materials Information. Products/services. Production process. A process is a combination of equipment, people and procedures that work together to produce products and services. Measurement process. Data.

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Measurement error

Roland Caulcutt Caulcutt Associates


Two processes
Two processes

Materials

Information

Products/services

Production process

A process is a combination of equipment, people and procedures that work together to produce products and services.

Measurement process

Data

All of your data comes from measurement processes.


Measurement error
Measurement error

Many people wish to believe that measurement error does not exist.

These people measure something once and assume that the measurement is “correct”.

They believe that taking a second measurement would be pointless.


Repeat measurements
Repeat measurements

15.6 15.8 16.0 16.2 16.4 Length

(cm)

Spread

The spread of repeat measurements quantifies the precision of the measurement process.


To assess precision
To assess precision

We could use: Spread = range of data

We use: Spread = 6sd.

15.6 15.8 16.0 16.2 16.4 Length

(cm)

Range

6 sd


Precision
Precision

If the measurements were made by the same person, using the same equipment, then:

The spread is a measure of repeatability.

If the measurements were made by different people then:

The spread is a measure of reproducibility or R&R.


VIM

International vocabulary of metrology (VIM) - 2008

The VIM defines repeatability and reproducibility.

If everyone adopted these definitions there would be no ambiguity.

Component manufacturers do accept them but many chemical manufacturers don’t. They prefer to focus on the difference that might exist if producer and customer measure the same delivery.


How good is your measurement process
How good is your measurement process?

The capability of your measurement process will depend on how you intend to use the data it produces.

You might use data to help you to:

  • Control a production process

  • Assess the capability of a production process

  • Improve a production process

  • Classify products or services

  • Settle customer/supplier disputes


Relative precision
Relative precision

To make your assessment of precision more meaningful you could relate it to the purpose of your measurement.

If you are measuring in order to classify product you could express your spread as a % of the specification.

If you are measuring in order to improve the production process you could:

  • Express your spread as a % of your product variation.

  • Calculate number of distinct categories


Classification of product

LSL USL Quality

Classification of product

Because of measurement error you may be left in doubt about the acceptability of the product or service.

Uncertainty

?????

Uncertainty

?????

Not

acceptable

Not

acceptable

Acceptable


R r as a of specification

LSL USL

LSL USL

LSL USL

LSL USL

R&R as a % of specification

R&R = 100%

R&R = 30%

R&R = 10%

R&R = 0%


GUM

Guide to the expression of uncertainty in measurement (GUM)

We make one measurement.

It is extremely unlikely that the measured value will be equal to the true value.

We report an interval in which we are confident that the true value will lie.

(True value) = (Measured value) + k(SD)


Bias

True value

Mean

15.6 15.8 16.0 16.2 16.4 Length

(cm)

Bias

Bias may be a bigger source of error than poor precision.



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