Measurement error
Roland Caulcutt
Caulcutt Associates
Two processes
Materials
Information
Production
process
Products/services
Measurement
process
Data
A process is a combination of equipment, people
and procedures that work together to produce
products and services.
All of your data comes from measurement processes.
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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.
3
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.
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To assess precision
We could use:
Spread = range of data
We use:
Spread = 6sd.
15.6
15.8
16.0
Range
16.2
16.4
Length
(cm)
6 sd
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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.
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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.
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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:
1. Control a production process
2. Assess the capability of a production process
3. Improve a production process
4. Classify products or services
5. Settle customer/supplier disputes
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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
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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
LSL
USL
Quality
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R&R as a % of specification
R&R = 100%
LSL
USL
LSL
USL
LSL
USL
LSL
USL
R&R = 30%
R&R = 10%
R&R = 0%
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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)
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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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Thank you for your attention.
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