iSixSigma

Gage RR

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  • #35063

    grover
    Participant

    Why do we say that Repeatability is “within variation scenario” ( I mean if one person is taking different reading with a verneir caliper and he is getting different readings everytime why we say that it is due to variation in the Vernier. It may be due to the person as well!!) and reproducibility is ” between variation scenario”.
    Can someone give me a brief example to understand  this theory of R&R.
    Regards

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    #97502

    XYZ
    Member

    let me try explaining you :-
    variation can either be process related or measurement related. Gage R&R helps in finding out if the variation in the data captured is due to measurement related and to what extent. R&R stands for repeatability and reproducibility. Repeatibility means if the same activity is performed by two different set of people what is the extent of variation and reprod. means if the same activity is performed by the same person again, what is the level of variation. e.g. if diameter of a bottle is to be measured and there are 2 people X and Y; then the difference in readings (for the same bottle)  when measured by X and Y individually would be explained by reproducibility and and the difference when measured by X alone or Y alone at different times by repeatibility.

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    #97503

    Nikhil
    Participant

    Naveen,
    Repeatibilty  : Is the same opeartor taking the reading
    Reproducibilty  : Is when different operators take readings
    So , ultimately reapeatibilty cannot show operator variaition

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    #97511

    Gabriel
    Participant

    What Nikhil said is wrong.
    In the context of r&R, “Repeatability” is the variation that exists in the measurment process even when all conditions are identical as far as we can tell. That is, several readings on the same patr, same instrument, same method, same operator, abut the same time.
    “Reproducibility” is the variation that exist in the measurement system when some conditions are changed, even when they are not supposed to cause a change in the measurement system’s behavior. Examples are two different operators or two different instruments of the same kind.
    An interesting point is that you will expect the result between, for example, two different operators to be different just by chance, even if there is no real difference between the operators. I mean, if you ask two persons to flip a coin ten times, you don’t expect that both will get exactly the same number of heads even if there is no difference in the technique, because you won’t expect the same result even if the experiment was repeated by the same person. That is, there is a part of the variation between operators that can be explained because the repeatability alone. If you find variation between operators beyond that point, that is reproducibility. That’s why you have a substraction under the square root in the formula for the reproducibility.
    With this in mind, it can be noted that, in language of SPC, reproducibility is a kind of variation that goes beyond the short term variation (which would be the repeatability), and hence it is a special cause of variation. The special cause can be a different trining, skill or technique, and it can be corrected. On the other hand, repeatability is variation due to common causes. It is not that common causes cannot be adressed, but you will not find 1 specific source to blame and fix. It is more a problem with the system itself.
    Hope it helped

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    #97627

    grover
    Participant

    In gage R&R, why individual measurement is expected to vary as much as +- (plus/minus) 3sigma.
    Why 6sigma (gage) is a good estimate of gage capability.
    Regards

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    #97632

    Gabriel
    Participant

    Because ±3sigma takes into account 99.73% of the population.
    It is the same reason why you use ±3sigma control limits, and why you use 6sigma to calculate Cp.
    You could use any other break-piont (like ±2 sigma or ±4sigma). But you will have to put it somewhere and anywhere you put it it will be as arbitrary as ±3sigma.

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    #98344

    ALEK DE
    Participant

    Repeatability & reproducibility both are evaluated while carrying out Gage R&R. Measurement system variation has two parts. Repeatability & reproducibility. Repeatability acconts for your Measurement instrument’s varaiation & reproducibility accounts for operator’s variation ( Operator & Operator by part). When we look at the result we look at total Gage R&R value for evaluating measurement system. Minitab calculates for total &  for repeatability & reproducibility separately. Looking at these you need to take action whether mesurement instrument is the trouble or operator ?
    One thing , while doing Gage R&R , you necessarily use more than one operators to have reproducibility else with one operator reproducibility is not applicable.
     
    Thanks!

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    #98380

    Jonathon L. Andell
    Participant

    I have a memory device that may be of assistance. Be warned, however, that some may find it slightly “politically incorrect.”
    Repeatability is like saying “I’m repeating myself.” Same operator, taking repeat measurements.
    Reproducibility is something that takes place between consenting operators.
    A tad indiscrete, but few forget afterward…

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    #98385

    Mario Perez-Wilson
    Participant

    My good friend Jonnathon Andell, long time no see.
    The way it goes is like this.
    Repeatability: If you are given a part and a gauge, can you obtain the same measurements everytime (REPEAT) you measure it. Can the gauge REPEAT the same measurement when you measure the same part. REPEATability is an estimate of gauge variation.
    Reproducibility: Now, if you give the same part and the same gauge to another person (operator) can s/he REPRODUCE the same values you got. Since nothing has changed (but the operator), then the variation has to be assumed to belong to the change in people or operator. REPRODUCibility is an estimate of operators variation.
    Now, this is just a SIMPLE explanation. If you want a complete explanation and the statistical method go buy a book about it.
    Mario Perez-Wilson (mpcps.com)

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    #98387

    Mike Carnell
    Participant

    Mario,
    Even longer time no see. Very smooth response.
    Headed for Lima on Saturday.
    Regards,
    Mike

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    #99463

    ophias
    Participant

    i need information on standard gauge repeatability index

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