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Difference Between Standard Deviation Overall and Within?

Six Sigma – iSixSigma Forums General Forums Tools & Templates Difference Between Standard Deviation Overall and Within?

This topic contains 3 replies, has 4 voices, and was last updated by  Rhb 2 months, 2 weeks ago.

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  • #208215

    SRattray
    Participant

    What is the difference between Standard Deviation (Overall) and (Within)? I’m currently using the formula for Overall, not sure of the formula for Within; or it’s purpose.

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    #208238

    Strayer
    Participant

    You can find the answer to this and most other questions by searching this site. It’s probably been discussed before. https://www.isixsigma.com/topic/difference-between-overall-stdev-and-within-stdev/

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    #208645

    Chris Seider
    Participant
    #208837

    Rhb
    Participant

    Answer is, standard deviation within is the short-term standard deviation from a sample, containing “normal” process variation. This standard deviation is used in Minitab control charts to calculate the control limits of a chart. Formula is, Average Moving Range / d2 for a sample size of 2. Standard deviation within is used to calculate Cpk.
    Conversely, standard deviation overall is the long term standard deviation of a Population, which contains both “normal” and “special cause” variation. Formula for this is the base standard deviation formula for a population and is used to calculate Ppk.

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