I-MR or X Bar and R Control Chart?

Six Sigma – iSixSigma Forums General Forums Tools & Templates I-MR or X Bar and R Control Chart?

This topic contains 1 reply, has 2 voices, and was last updated by  Robert Butler 1 week ago.

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    Karim Mohsen

    If I had about 30 samples obtained from one batch produced in certain time and under same condition, which is better to use I-MR chart of X bar & R chart?

    And which chart should i use if i had 30 results from 30 different batches produced in several days ?


    Robert Butler

    If by a batch you mean a single run such as contents of one reactor vessel then the answer is neither because those samples are repeated measures – not independent measures. If you try to build any control chart with that kind of data the control limits will be too narrow because the sample-to-sample variability within a single batch is going to be less (and possibly much less) than batch-to-batch variability.

    As for the second question concerning 30 results from 30 batches – the question you need to answer is this: Is there any obvious subgrouping of the measures? If there isn’t then forcing what amounts to single measures into some artificial grouping is of little value. In all of the cases I ever dealt with where the output was a batch of material I used IMR charts and never had a problem.

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