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which is the right control chart

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  • #38460

    AP
    Participant

    guys Im just starting out with this project of implementing SPC on the shop floor at my company. I am in charge of design, set up and implementation. I want to make sure we are using the control chart. Most of the time we will have about 50 sets of measurement data with 3-5 subgroups each set. I was wondering if XbarR is a good choice or XbarS. Can someone help me out here?
    Thanks in advance

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    #115020

    Harkanwal
    Participant

    Xbar and R charts are typically used to track the process level and process variation for samples of size 8 or less, while Xbar & S Charts are used for larger samples.
    If you will be working with 50 sets of measurement data Xbar & S Chart is a better choice.
     
    Hope this helps

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    #115024

    S
    Member

    Almost.
    X-bar R charts are used if your subgroup size is 10 or fewer.  Larger than 10 you can use the X-bar S.
    If your subgroup size is 3-5 (let’s say 5) and you have 50 measurements, you would have 10 subgroups charted on an X-bar R chart.

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    #115032

    FTSBB
    Participant

    Straight from the mouth of MiniTab:
    “Xbar and R charts are typically used to track the process level and process variation for samples of size 5 or less, while Xbar and S charts are used for larger samples.”
    MiniTab (or other stat software) will show you how the values are calculated.  Take the above with a grain of salt – think about HOW the limits are calculated and how sensitive you really want your charts to be.  Make sure you understand the difference between S and R and how the control limits are calculated.

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    #115038

    Mikel
    Member

    All of these are just rules of thumb. Go look for yourself the acccuracy of R-bar/d2 vs. s-bar.
    You will find that the error when using s-bar for samples of 2 is very big compared to r-bar. Other than that the error is a wash. Period, end of discussion. Shewhart and his boys knew what they were doing when they came up with the values for d2 (and they got values for up to n=25 at least).
    Conclusion, don’t use s when n=2. Otherwise it does not matter.

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    #115483

    Sezgin Ezgi
    Member

    I would also like to mention CUSUM charts which i believe very effective.

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    #115501

    emo
    Participant

    KISS.  You can use either one.  Just know what your looking at.

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    #115503

    MW Richardson
    Participant

    You are asking very basic questions. There is nothing wrong with that. If that is the case, and you haven’t done so already, you should consider taking a basic course in SPC. That being said, you are definitely on the right track. I have seen many projects fail to deliver the results they should have because of a fundamental lack of understanding of the process’ central tendency and variation over time.
    MW Richardson,
    Sr. Q.A. Eng.

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