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How to round off percentage uncertainties?

  1. Mar 23, 2012 #1
    How to round off percentage uncertainties???

    Here's is an example question:

    Find the percentage of uncertainty in (225.7+/-0.5)mm.
    I know that to find out percentage, it's simply 0.5/225.7 x 100 = 0.22153088...%

    Now, my question is, how much do I round off for this percentage uncertainty? Should I just round it off to one significant figure? Or Is there a certain rule?
  2. jcsd
  3. Mar 23, 2012 #2
    Re: How to round off percentage uncertainties???

    strictly speaking you cannot use more sig figs than the smallest you are given/calculate......that is what the word 'significant' means (If they are not significant then they are insignificant !!!!)
    Your error is quoted as +/-0.5mm which means 1 significant figure so I would say round off to 0.2%
    I would challenge anyone dealing with error analysis to justify that this should be given to 2 or 3 sig figs.
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