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Log (x) and error in x

  1. Oct 2, 2008 #1
    Hello,

    I have a column of 'x' values, each with an error 'err_x' serving as error bars.

    My question is: If I need to plot 'log(x)', Do I need to make any changes to 'err_x' to reflect the log operation before plotting them as error bars ?

    Thank you in advance

    Lorna
     
  2. jcsd
  3. Oct 2, 2008 #2

    CRGreathouse

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    I would plot log(1 + error) for positive error and -log(1 - error) for negative error. That way error is on the same scale.
     
  4. Oct 2, 2008 #3
    What is the significance of the '1' ? and why not just log(error)?
     
  5. Oct 3, 2008 #4

    CRGreathouse

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    Why don't you pick some errors in -1 < e < 1 and see for yourself?
     
  6. Oct 5, 2008 #5
    Thank you, althought I still dont understand why "1" not another number
     
  7. Oct 6, 2008 #6

    CRGreathouse

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    It's the only number that will make the error bars tend to 0 as the error approaches 0. With a smaller value very small errors won't show up at all (calculation error trying to take a negative log); with a larger value errors won't ever get closer than a certain distance from the point no matter how small the errors are.
     
  8. Oct 6, 2008 #7
    Now I see. Thank you SO MUCH!!!
     
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