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Control Limits too Narrow

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  • June 11, 2008 at 11:53 am #50264

    chhabra
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    I am doing a P-chart but getting control limits that are too narrow….all the data points are out of control. The avg p is 0.06019, UCL is 0.06073, LCL is 0.50965.
    What’s going wrong ? My sample size per day is high…actually 6,00,000 to 9,00,000. Is that the reason ? What shd be the right approach to solve this riddle and come to reasonable decisions ?
     

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    June 11, 2008 at 12:51 pm #172737

    DaveS
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    Amit,
    Please repost with clarified numbers.
    I think you mean LCL = 0.050965 not 0.50965. What is the sample size used to calculate these limits? I’m not familiar with a convention where we have significant digit comma two more digits comma three more digits? Is it six hundred thousand? Six million? Sixty thousand?
    Your calculations of the control limits are wrong, regardless. How are you doing them?
    Your UCL and LCL are not symmetric around the average if the LCL is taken as 0.050965.
     
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    June 11, 2008 at 1:38 pm #172740

    luke skywalker
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    Yes, your sample size is several order of magnitude too high for a p-chart. It can adequately handle samples around 500 but fades as the sample sizes increase from there.
    There is a set of test criteria to assess this: np>5 AND n(1-p)>5
    If you satisfy both of those, the stats books suggets that your binomial data approximates the normal enough for you to use an I-mR chart. The observations plotted there would be th eindividual sample proportions.
     
    Cheers!

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    June 11, 2008 at 5:21 pm #172746

    chhabra
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    Thanks Luke. The I-chart looks good.

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