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R&R
Studies
by Paul
A. Keller, CQE, CQA
As discussed in last month's
column on Calibration Studies, a Calibration
provides just one element of measurement uncertainty for the measurement
system. Additional sources of uncertainty exist that must be separately
analyzed. An R&R Study provides a technique for estimating two components:
Repeatability and Reproducibility.
Repeatability
is the ability of the measurement system to provide consistent readings
when used by a single technician or operator on a given piece. Repeatability
is sometimes referred to as Equipment Variation.
Reproducibility is the ability for multiple technicians or operators
to achieve consistent results, and is sometimes called Appraiser Variation.
At first glance, many
of us would expect any reliable measurement system to have zero error
associated with Repeatability and Reproducibility. We purchase inspection
equipment, and if there is error in the measurements, there must be something
wrong with the equipment. But R&R Studies measure much more than the
error of the particular gage: they measure the error in the measurement
system, of which the particular gage is one component.
When conducting an R&R
Study, it is imperative to use production parts, or parts that are similar
to production parts (such as when performing pre-production evaluations).
You must not use reference standards, since the ability of the measurement
system to precisely measure the standards is not a good indicator of whether
the system will precisely measure parts in production. Remember the purposes
of the measurement system: to evaluate the suitability of given product
for release to (internal or external) customers, and/or to evaluate the
stability (i.e. statistical control) and capability of the process producing
the parts.
Using production parts
allow us to include error associated with measurement fixturing (used
to hold the part or gage during measurement), as well as variation within
the production pieces themselves. Each of these generally contribute to
increased Repeatability errors, since all the operators contend with these
problems. However, it might be that select operators have improved techniques
for dealing with the poor fixturing or the within-piece variation, so
that their Repeatability error is less than other operator's. Recognizing
this, when it occurs, can lead to system improvements, as we will see
in later articles.
Of course, these techniques
are not limited to mechanical inspections, nor to material parts. They
can also be applied to optical inspections, chemical analyses, or any
other measurement system. In chemical analyses, sample preparation may
be a critical component of the measurement. Just as we use production
parts above, this preparation should be done using actual product, so
that sample preparation is done using 'real-world' conditions.
R&R Studies are conducted
by obtaining a sample of parts, and having representative operators inspect
each part (called a piece) multiple times (each inspection is
termed a trial). A typical study involves having 3 operators
measure 10 pieces 3 trials each. The number of pieces, operators, and
trials can vary from case to case, but we need multiple trials to estimate
Repeatability and multiple operators to estimate Reproducibility. Multiple
parts allows us to obtain better estimates of Repeatability and Reproducibility,
as well as to estimate Part Variation.
When we conduct the R&R
Study, keep these things in mind:
- use production pieces
(see above)
- randomize the order
of the pieces presented to each operator
- number each part (example
1-10), but try to keep this 'blind' to the operator (some operators
have astounding memories!)
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