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Control Charts Application

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  • #53698

    Rasheed
    Participant

    if i need to make X chart then how can i calculate the range with sample size one? is it possible to make control charts with sample size 1…

    If yes how? if no please provide any solution.

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    #191146

    HopeOverExperience
    Participant

    Why do you need a X bar chart if you’re sample size is 1 isn’t this an IMR?

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    #191147

    Rasheed
    Participant

    its not X bar, it X for indivisual measurement and range. please advice,

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    #191148

    Robert Butler
    Participant

    Your situation is that of a control chart with subgroup size of one. Obviously you don’t have subgroup ranges. What you have to do is estimate short term variation by taking the average of the moving range which is based on successive differences between the individual values.

    Page 48 of Understanding Statistical Process Control 2nd Edition – Wheeler and Chambers has the details.

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    #191149

    Rasheed
    Participant

    Thanks for replying.

    if i will go on moving average then UCL and LCL of range will change continuously as no. of readings are increasing each time.

    further can u send me the link of the below above mentioned book so i will study online and try to find the solution.

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    #191153

    Robert Butler
    Participant

    No, that isn’t the way it works. What you are doing is taking averages of successive differences therefore for ranges based on a subgroup size of 2 d2 = 1.128 – it doesn’t change. Similarly D4 = 3.268 and because of the subgroup size of 2 it doesn’t change either. The text in the book is too long to attempt to quote here – you need to get a book on control charting and read the appropriate section.

    As for the book – I don’t know of any online version – If you can’t find the one cited I’d recommend going to a library and finding a book on process control – the topic will be control charts for individual measurements – you might also be able to find something on this topic online somewhere.

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