Measurement system analysis (MSA) uses scientific
tools to determine the amount of total variation is from the measurement
system. An objective method to assess the validity of a measurement
system and minimize these factors that could excessively contribute to
the variation in the data.
Confirm that the measurement used to collect the data
is valid. Goal is to quantify the equipment/process variation and
appraiser variation and the total measurement system variation.
The following areas components of measurement error
needs to be studied and quantified before establishing capability of a
process making decisions from the data.
REPEATABILITY & REPRODUCIBILITY (Gage R&R)
This is often a very time consuming component of the
project and can slow the team’s quick progression through the process.
Continue to focus on low hanging fruit that may be
momentum sustainers and work vigorously through the MSA process. Most of
this can be done by the GB/BB outside of the team meetings and results
shared with them when complete.
Accuracy / Bias
The difference from the true value and the value
from the measurement system. Accuracy represents the closeness to a
defined target. Precision is different than accuracy and is covered in
Gage R&R under Repeatability.
For best accuracy of the data:
1) Accept all data as it is collected. Assigning
special cause and scrutinizing the data can come later.
2) Record the data at the time it occurs.
3) Avoid rounding off the data, record it as it is.
4) On the data collection plan, record as many details
around the data such as the exact source, machine, operator, conditions,
collector’s name, material, gage, and time. Record legibly and
The data should be screened for misplaced decimal
points, duplicate data entries by mistake or improper recording
procedure, missing date points if frequency is important, and other
obvious non-representative data.
5) Verify the gage is accurate. If using a weigh
scale, verify it with a known and calibrated weight. Use gage blocks for
calipers or micrometers. Use hardness blocks to verify hardness testers.
Resolution / Discrimination
The goal is to have at least 5 distinct values or
categories of readings.
Adhere to the 10-bucket rule. If your measurement
system requires measurements to the hundredths (x.xx), then divide that
by 10. Collect and record the data to the nearest thousandths (x.xxx).
The measurement system shall be sensitive to change and capable of
The lack of resolution will not allow a measurement
system detect change. If you are measuring the downtime and using
measurement to the nearest hour and most downtime is less than an hour
then most of the reading will either be a 0 (for 0 hours) or a 1 (for 1
However, using a stop watch and recording data to the
nearest minute will provide 60x more resolution and allow better
distribution of data points, more variety of data, with fewer repeat
measurements. You could have 60 different readings. Actually recording
the nearest 6 minutes would satisfy the 10-bucket rule, but it is a
guide to help ensure resolution in the measurement system.
This part of the MSA is usually the easiest to fix
such as finding a micrometer, caliper, hardness tester that can capably
read to the next nearest decimal.
Try acquiring a larger samples size, with the idea
that some of these may create new observations or measurements.
Measure to as much resolution as possible and
When gathering data only collect with the
acceptable limits where there is proven linearity. This is a test to
examine the performance of the measurement system throughout the range
of measurements. For example, does the bathroom scale perform the same
when weighing a pet of 10 lbs to a man of 250 lbs?
Stability of a
measurement system is analyzed using control charts. Ensuring the
measurements taken by appraiser(s) for the process is stable and
consistent over time.
SPC Charts use a
variety of tests to determine stability. Many software programs will
have these as options to include when analyzing data and will even
indicate the point(s) and test that each failed.
Some of the corrective measures once again include
Standard Operating Procedures. Each appraiser should measure the same
way every time over a long period of time and each appraiser should
measure the same way as all the others. Recall that special causes can
also occur with the process control limits and these must be given
corrective action before proceeding to validate the measurement system.
Variable Gage R&R
In a variable Gage R&R there are generally two to
three operators appraisers with 5-10 process outputs measured by each
appraiser. Each process output is measured 2-3 times by each
operator.Depending on the cost and time involved you can add more
appraisers and measurements and replications.
When performing the replicated appraisals it is
critical that the measurement are randomized so that no patterns or
predictability can be entered in by the appraiser. This bias will
mislead the team and create a useless Gage R&R.
For example, an appraiser may remember the 7th part
that was measured was borderline and made a decision to give it one
measurement. He/she may have spend a lot of time of that part and if the
2nd round of measurements are not randomized, that person will remember
the measurement (appraisal) they gave it on the first round.
So, move the parts around each repeat set of
measurements. However, the parts must be indentified so the person
entering the data into the statistical software enters the reading under
the correct part.
Four Criteria in Variable Gage R&R
The following four areas will
be asssessed. A statisical software program will produce these values
once the data is properly entered. The GB/BB will be responsible for
finding these values and determining whether each passes and if the
entire measurement system is adequate to determine process capability.
Process capability can not be determined with reliablity if the
measurements (the data) is suspect.
1) % Study Variation is based on
Tolerance is based on USL and LSL
3) % Contribution is based on variance
number of distinct categories
based on process variation
Ideally, all four categories should be in the GREEN
zone. Examining the visual aids below shows commonly used judgement
criteria for each category.
2) % Tolerance
Shown below is an example of a % TOLERANCE
calculation. In this case we are using 3 appraisers measuring 6
This study shows the measurement error as a percent of
tolerance in short period of time. It oncludes both repeatability and
reproducibility, can not be separated.
5.15 Study Variation = 99% (constant)
The TOP TABLE at the top is a part of the d2
distribution. This value is a constant that is found by looking at the
column with 3 appraisers and going across with the row with 6 parts. In
this example the d2 value is 1.73.
The LOWER TABLE shows that actual measurements that
each of the appraisers cam up with using their variable gage. The range
of the three measurements for each part is shown on the right. Then the
average range is shown (=0.69) and this is carried on to the Gage Error
To convert this gage error of 2.05 to a percentage
of tolerance multiply by 100 and divide by the process tolerance for the
The process tolerance is the difference in the
specification limits. For example, if the USL is 27 and the LSL is 2,
then the tolerance is 25.
With the tolerance being 25, then:
Referring back to the
RED/YELLOW/GREEN criteria display for % TOLERANCE, it shown that 8.2% is
a passing value and this part of the Variable Gage R&R is successful.
Ability of one appraiser to get the same result and
another appraiser or the ability of all appraisers to get the same
results AMONG each other.
To optimize reproducibility in ATTRIBUTE Gage R&R:
1) Create visual aids, templates, definitions, or
other specific criteria for each to meet a certain rating, value, or
appraisal. Pictures of good, bad, in the middle, and colors, will help
each appraiser standardize their response, improving the
Note: If these corrective actions are needed to pass
the Gage R&R, it should be instituted as a formal work instruction and
everyone involved throughout the company or plant should adhere to same
To optimize reproducibility in VARIABLE Gage R&R:
1) Create a Standard Operating Procedure with visual
aids and definitions. When using humanly subjective "touch" devices such
as micrometers and calipers it is important that all appraisers
"squeeze" the same amount. Too little or much pressure at higher levels
of resolution can be enough to alter the Gage R&R.
2) Visual aids also help. Even when using an optical
comparator to get a higher resolute data point there is subjectivity
where to place the template or the starting and end point(s) on the
shadow. Pictures of acceptable and non-acceptable will help reduce this
This describes the ability for an appraiser to repeat
his/her measurements each time when analyzing the same part, unit, etc.
In destructive testing (such as tensile testing) these reading will not
be possible and some statistical software programs have options to
select for destructive testing.
The goal is to have an appraiser repeat unit readings
at least three times. The person administering the test should randomize
the sequence each time to prevent and patterns and bias (the appraiser
may remember or try to remember what a measurement was and tend to alter
real measurements to get the Gage R&R to pass). It is important for the
administrator to record carefully to ensure readings correlate the
correct part/unit each time.
Avoid writing down measurements and then typing them
into a statistical program. The fewer times measurements are recorded
and copied the lower the risk for human error to add even more variation
and possibly fail (or pass) the Gage R&R when it shouldn't have.
Precision is the ability to have the same repetitive
result (or appraisal in this case). Visually, it means that all your
shots of an arrow are very close to one another. It does not mean that
they are near the bulls eye. In other words, it does not mean that your
shots are accurate.
If your shots are accurate and precise, then they are
tight circle centered around the target. It is also possible to be
somewhat accurate without being precise. You may have several shots all
around the bulls eys (target) but they may be scattered around it. If
you take a look at the group the center (mean) may be the bulls eye but
the shots are not in control or precise. In others words, there is a lot
of unpredictability or variation.