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Estimate for Population Standard Deviation w/o samples

Discussion in 'SPC - Statistical Process Control' started by Danielle Beard, Oct 25, 2019.

  1. Danielle Beard

    Danielle Beard New Member

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    For a particular process that is in good statistical control, the mean standard deviation value, sbar, is 0.795. Calculate the estimate for the population standard deviation (the Shewhart standard deviation) if the sample size is 6.

    Can someone explain to me how to figure this out? I know how to do a regular standard deviation, mean, and mode with samples. I cannot figure this out for the life of me.......

    I just need someone to tell me how to get the calculations without having the samples?
     
  2. Bev D

    Bev D Moderator Staff Member

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    Is this a homework question or a work question?
     
  3. Danielle Beard

    Danielle Beard New Member

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    Homework. I am trying to understand how to get the estimate for the population standard deviation. I don't understand how to get it without the samples. I don't need an answer to the question, only how to calculate it.

    Are we not allowed to post homework questions?
     
  4. Bev D

    Bev D Moderator Staff Member

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    Of course homework questions can be posted but our answers will be different - as you say we will give your help to answer the question on your own - we will not give you the answer.

    Sbar is translated to the population standard deviation by dividing by a factor called C4. The estimate of the standard deviation from a series fo small samples is ‘biased’ and therefor emyust be corrected by the factor. The factor can be found in any table of Control Chart factors - it’s based on sample size. Or it can be calculated directly. If this is a homework question I would assume that the factor is in your text book. If not a simple google search will give you the table and/or the factor...
     
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  5. tony s

    tony s Well-Known Member

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    Here's the table:

    upload_2019-11-11_9-19-46.png
     

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