iSixSigma

Control Limits too Narrow

Six Sigma – iSixSigma Forums Old Forums General Control Limits too Narrow

Viewing 4 posts - 1 through 4 (of 4 total)
  • Author
    Posts
  • #50264

    chhabra
    Participant

    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 ?
    Β 

    0
    #172737

    DaveS
    Participant

    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 itΒ 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.
    Β 
    .

    0
    #172740

    luke skywalker
    Participant

    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.
    Β 
    Cheers!

    0
    #172746

    chhabra
    Participant

    Thanks Luke. The I-chart looks good.

    0
Viewing 4 posts - 1 through 4 (of 4 total)

The forum ‘General’ is closed to new topics and replies.