# MSA Using MTB Questions

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• #42706

SigmaGuppy
Member

1.  If you have two categories of variation – accuracy and precision – and the Gage R&R (continuous) provides repeatability and reproducability measures (both which represent precision) where is accuracy represented in the MTB session info?
2. Where does a test-retest fit into the Gage R&R study?

Same operator, same part, same device, multiple measures
Calculate and compare Sample Avg to Std (accuracy) and
Sample std deviation to tolerance (s < 1/10 of tolerance for precision) into the Gage R&R study?
3.  Can you / Do you prioritize the the %Tol, %Study, %Cont measures, or is it a “fail one, fail all” (based on Rules of Thumb)
Thanks!  Sorry for so many questions.

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#134975

Neeraj Katare
Participant

For Accuracy, pl use Minitab Option-Gage Linearity and Bias Study
Stat > Quality Tools > Gage Study > Gage Linearity and Bias Study
Gage linearity tells you how accurate your measurements are through the expected range of the measurements. It answers the question, “Does my gage have the same accuracy for all sizes of objects being measured?”
Gage bias examines the difference between the observed average measurement and a reference or master value. It answers the question, “How biased is my gage when compared to a master value?”

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#134980

SigmaGuppy
Member

Thank you so much!

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#134983

BTDT
Participant

1

questions, MSA is a large and very important part of every project.You will be doing three
tests. At each test, if you dont pass, then go to jail, do not collect \$200.First test  resolution
checkCan you even hope to read
the gauge to the precision you require? The readout of the gauge must divide
the tolerance window into 10 divisions to be acceptable. Would a sundial be
adequate to measure time while running a production line? You cant read
seconds. No, you cant use it. The motion of the planets is a very well
understood phenomenon, but you just will never be able to read seconds from a
sundial.Second test 
test-retestYou can find the pure
equipment variation by having one operator, one gauge, and one part. Take (say)
15 measurements, find the standard deviation and compare it with the tolerance
width. This will give you pure gauge error. By the way, if you are measuring
one part over and over, why not measure a known standard? You purchase these
for exactly this purpose. The difference between the mean of the 15
measurements and the number on the calibrated standard will give you the bias
of the gauge. This is not found as a separate Minitab function, you use the graphical
summary to make the calculations. There is no rule-of-thumb for judging whether
the bias is acceptable or not. This is because it is so easy to correct. The
device can be re-zeroed or the bias can be corrected by subtracting it from all
gauge linearity and biasIf you are going to use the
gauge in a variety of settings with a large range of values, then you may
consider using a range of standards and conducting a gauge linearity and bias
study. This is a bit more involved than the test-retest because it requires a
range of standards that cover the range of interest. It will also allow you to
determine the bias as a function of the size of the measurement.Third test  Gauge
R&RThe big meal deal, using
actual instruments in field conditions by the workers who will be using them.
It is best to use a sample of parts from real production.Minitab does divide the
precision into the two categories of operator and equipment. This is the reason
behind all the different combinations of operator and part numbers. By looking
at the variation in data from one operator measuring the same part multiple
times, then you can quantify the pure error. This number will be similar to the
one from the test-retest, but remember the test-retest was done under ideal
conditions, the Gauge R&R is done under field conditions. The equipment
error here is likely to be larger than the one determined during the
test-retest.To interpret the results
you can treat the rules-of-thumb more as guidelines.If the total Gauge R&R
(the combination of equipment error and operator error) is less than 10% of the
tolerance window, then it is considered OK. The %Study variation always totals
100% and merely allows you to Pareto the sources of variation. If you only see
%Study variation without the %Tolerance variation, you forgot to enter a value
for the tolerance window found under options.If the process variation is
very large, you will have a selection of parts where you have a lot of them
outside the specification limits. In this case you can have the number of
distinct categories rule-of-thumb indicate the gauge is acceptable, but the
Gauge R&R may be much larger then 10% of the tolerance window. This is
because when the process is really bad, then even a poor gauge can tell whether
a part is way-too-large, too-large, OK, too-small, and way-to-small (5
categories) while not telling you too much about where the parts are with
respect to the Upper-OK limit and Lower-OK limit.I do see a lot of people
neglect the test-retest step, because they get the same information out of the
Gauge R&R.Cheers, BTDT

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#134985

SigmaGuppy
Member

BTDT, thanks so much….Per usual, that was VERY helpful …… Cheers!

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#134991

BTDT
Participant

SG:You’re welcomeCheers, BTDT… and I got post 90,0000 too

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#134992

Dartman_undersiege
Participant

Hey, BTDT..
Was this coincedence or you do it on purpose?  I mean you being in 90,000 post.
Anyway, I’m next to you. 90001 perhaps….

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