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Math Help - Shewhart Control Charts

  1. #1
    ivanmad
    Guest

    Shewhart Control Charts

    I would like to group data points of which I have several hundred thousand over a year's time. the data of interest is time per call. I have this information for every call. I want to look at it by week. If I use an Xbar R chart I would need to have the same sample size for every week. I won't because the call volume is variable. An I-MR chart would have too many data points. Is it valid to use an I-MR chart by putting down an average for each week as the individual data points?
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  2. #2
    Newbie
    Joined
    Jan 2006
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    Puerto Rico
    Posts
    9
    It is valid, but you must recognize that the variability will differ from week to week due to the varying sample size. Another approach is to scale the data for each week by the sqrt(sample size) to stabilize the variance. In either case, consider the assumptions required for the type of analysis you plan to do after you have the time series.
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