ANOVA Gauge R&R
ANOVA Gauge R&R (or ANOVA Gauge Repeatability & Reproducibility) is a Measurement
Systems Analysis technique which uses Analysis of Variance (ANOVA) random effects model
to assess a measurement system.
The evaluation of a measurement system is not limited to gauges (or gages) but to all types of
measuring instruments and test methods.
2 How to perform a Gauge R&R
3 Common Misconceptions about GRR
4 External References
5 See also
ANOVA Gauge R&R measures the amount of variability induced in measurements that comes
from the measurement system itself and compares it to the total variability observed to
determine the viability of the measurement system. There are several components affecting a
measurement system including:
Measuring instruments, the gauge or instrument itself and all mounting blocks,
supports, fixtures, load cells etc. The machine ease of use, sloppiness among mating
parts, quot;zeroquot; blocks are examples of sources of variation in the measurement system;
Operators (people), the ability and/or discipline of a person to follow the written or
Test methods, how to setup your devices, how to fixture your parts, how to record the
Specification, the measurement is reported against a specification or a reference value.
The range of the specification does not affect the measurement, but is an important
factor affecting the viability of the measurement system.
Parts (what is being measured), some items are easier to measure than others. A
measurement system may be good for measuring steel block length but not for
measuring rubber pieces.
There are two important aspects on a Gauge R&R:
Repeatability, the ability of the device to provide consistent results. It is a measure of
the variability induced by the system if the same operator measured the same part using
the same device repeatedly.
Reproducibility, the variability induced by the operators. It is the variation induced
when different operators (or different laboratories) measure the same part.
It is important to understand the difference between Accuracy and precision in order to
understand the purpose of Gauge R&R. Gauge R&R only address how precise a measurement
Anova Gauge R&R is an important tool within the Six Sigma methodology, and is also a
requirement for Production Part Approval Process (PPAP) documentation.
How to perform a Gauge R&R
The Gauge R&R is performed by measuring parts using the established measurement system.
The goal is to capture as many sources of measurement variation as possible, so they can all be
assessed and addressed. Please note that the purpose is not to quot;passquot;. A small variation reported
on a GRR may be because an important source of error was missed during the study.
In order to capture reproducibility errors, multiple operators are needed. Some (ASTM E691
Standard Practice for Conducting an Interlaboratory Study to Determine the Precision of a Test
Method) call for at least ten operators (or laboratories) but others use only 2 or 3 to measure the
In order to capture repeatability errors, the same part is usually measured several times per
In order to capture interactions of operators with parts (e.g. one part may be more difficult to
measure than other), usually between 5 and 10 parts are measured.
There is not a universal criteria for minimum requirements for the GRR matrix, being up to the
Quality Engineer to assess risks depending on how critical the measurement is and how costly
they are. The 10x2x2 (10 parts, 2 operators, 2 repetitions) is common, although it has very few
degrees of freedom for the operator component.
It is used as part of the Six Sigma DMAIC process for any variation project.
it is imperative to use production parts, or parts that are similar to production parts (such as
when performing pre-production evaluations).
Common Misconceptions about GRR
Need only one GRR per family of gauges. It is usual to say quot;There is an acceptable
GRR for this caliperquot;. This statement is false, as a GRR is for the measurement system,
which includes the part, specification, operator and method. As an example, measuring
a steel block with a caliper may be achieved with a good precision, but the same caliper
may not be suitable to measure soft rubber parts that may deform while it is being
The GRR will not pass using parts, so it has to be done with standard weights and
blocks. The GRR done in this way will assess the precision while measuring standard
weights. The device might not be suitable to measure that specific type of parts. If the
part quot;changesquot; while being measured, this has to be counted as a measurement system
Need to report on PPAP documentation GRR results for everything that is
measured. This is not necessarily a requirement. The Quality Engineer usually makes
an educated assessment. If the characteristic is critical to safety, a valid GRR is
required. Instead, if there is enough understanding that some particular part is easy to
measure with acceptable precision, a formal GRR is not required. Customers may ask
for additional GRRs during PPAP reviews. Knowing that a GRR is not good and still
uses the measurement system does not make sense. This is like using bent calipers to
get measurements, you get a number but it does not mean anything.
Performing a GRR is very expensive. In order to perform a GRR usually a number of
parts (sometimes between 5 to 10) is required to be measured by at least 3 operators
(some suggest ten or more) 2 to 3 times. So the measurement costs are the ones
associated with those additional measurements. For simple devices this may not be very
costly, and the results is a known measurement error that can be used to assess all
measurements subsequent to that. The costs can be higher for destructive testing.
GRRs must be within 10% to pass. There are AIAG guidelines for GRR errors
relative to the specification, and what to report on a PPAP process. The final call is
between the supplier and customer, and it is a function of the critically of the
characteristic and the assessed measurement error. GRR is a tool that helps making this
assessment, but it does not gives you the answer.
Gauge Repeatability and Reproducibility (GR&R)
• Gauge Repeatability and Reproducibility, or GR&R, is a measure of the capability of a
gauge or gage to obtain the same measurement reading every time the measurement process
is undertaken for the same characteristic or parameter. In other words, GR&R indicates the
consistency and stability of measuring equipment. The ability of a measuring device to provide
consistent measurement data is important in the control of any process.
• Mathematically, GR&R is actually a measure of the variation of a gage's measurement, and not
of its stability. An engineer must therefore strive to minimize the GR&R numbers of his or her
measuring equipment, since a high GR&R number indicates instability and is thus undesirable.
• As its name implies, GR&R (or simply 'R&R') has two major components, namely, repeatability
and reproducibility. Repeatability is the ability of the same gage to give consistent
measurement readings no matter how many times the same operator of the gage repeats the
measurement process. Reproducibility, on the other hand, is the ability of the same gage to
give consistent measurement readings regardless of who performs the measurements. The
evaluation of a gage's reproducibility, therefore, requires measurement readings to be acquired
by different operators under the same conditions.
• Of course, in the real world, there are no existing gages or measuring devices that give exactly
the same measurement readings all the time for the same parameter. There are five (5) major
elements of a measurement system, all of which contribute to the variability of a measurement
process: 1) the standard; 2) the workpiece; 3) the instrument; 4) the people; and 5) the
All of these factors affect the measurement reading acquired during each measurement cycle,
although to varying degrees. Measurement errors, therefore, can only be minimized if the
errors or variations contributed individually by each of these factors can also be minimized.
Still, the gage is at the center of any measurement process, so its proper design and usage
must be ensured to optimize its repeatability and reproducibility.
• There are various ways by which the R&R of an instrument may be assessed, one of which is
outlined below. This method, which is based on the method recommended by the Automotive
Industry Action Group (AIAG), first computes for variations due to the measuring equipment
and its operators. The over-all GR&R is then computed from these component variations.
• Equipment Variation, or EV, represents the repeatability of the measurement process. It is
calculated from measurement data obtained by the same operator from several cycles of
measurements, or trials, using the same equipment. Appraiser Variation or AV represents the
reproducibility of the measurement process. It is calculated from measurement data obtained
by different operators or appraisers using the same equipment under the same conditions.
The R&R is just the combined effect of EV and AV.
• It must be noted that measurement variations are caused not just by EV and AV, but by Part
Variation as well, or PV. PV represents the effect of the variation of parts being measured on
the measurement process, and is calculated from measurement data obtained from several
• Thus, the Total Variation (TV), or the over-all variation exhibited by the measurement system,
consists of the effects of both R&R and PV. TV is equal to the square root of the sum of
(R&R)2 and (PV)2 square, i.e.,
• TV = √ (R&R)2 + PV2.
• In a GR&R report, the final results are often expressed as %EV, %AV, %R&R, and %PV, which
are simply the ratios of EV, AV, R&R, and PV to TV expressed in %. Thus,
%EV=(EV/TV)x100%; %AV=(AV/TV)x100%; %R&R=(R&R/TV)x100%; and
%PV=(PV/TV)x100%. The gage is good if its %R&R is less than 10%. A %R&R between 10%
to 30% may also be acceptable, depending on what it would take to improve the R&R. A
%R&R of more than 30%, however, should prompt the process owner to investigate how the
R&R of the gage can be further improved.
Repeatability is the variation in measurements observed when one operator repeatedly
measures the same characteristic in the same place on the same part with the same
gauge: (i.e. variation in measurements under identical conditions). (See
A limit within which agreement may be expected 95% of the time between two test
results obtained in essentially the same conditions and from the same homogeneous
sample of material.
Repeatability is the inherent variation within the measurement instrument (gauge) and
is represented by repeatability, which is the standard deviation of the measurement
Reproducibility is the variation in average measurements due to factors other than the
These factors include, but are not limited to, operators, different gauges, temperature,
humidity, and part fixturing technique. (See Repeatability)
Reproducibility is the variation due to the measurement system.
R & R studies can partition reproducibility into these component parts.
The partitioning can be used to decide where the problem areas are and improve the