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Standard Deviation

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  • #32968

    mohit
    Participant

    Why do we need Standard Deviation??

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    #88604

    Summerfield
    Participant

    Standard deviation is the measurment of your process variation.

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    #88605

    mohit
    Participant

    Why do we need to use standard deviation to find out the variation when their are other ways of doing it??

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    #88616

    Zilgo
    Member

    Why do we need to use any other method when there is standard deviation?
    As long as it makes sense and can be interpreted, then there is nothing wrong with using another measure.  SD is just very common and applies to all statistical testing.

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    #88621

    Keshri
    Participant

    Actually its a good point. Standard deviation might not be a good measure if the data is not normally distributed. eg a span or range is a good measure when its a long tail distribution and Stability factor Q1/Q3 is a measure of variation  when data is skewed. We need to see the distribution of data before deciding the rightr measure of variation. A normal probalbility plot would help.
    Also Range is good measure of variability whre we have few data points ( like destructive testing). Range is also used to measure Repeatability error in measurement system analysis.
    nayan

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    #88623

    Gabriel
    Participant

    Usually, Range is just an intermediate step in the way to Standard Deviation. For example, in r&R studies or in proces capability studies from Xbar-R charts you get each R for each sugroup, then calculate the average range, and then calculate the standard deviation as Rbar/d2. Finally you use that standard deviation (not the range itself) to calculate the r&R or Cpk metrics.

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