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Gre Problem # 40

  1. Aug 31, 2004 #1
    Dear members,
    here is a gre problem that I couldn't know how to tackle, any effort will be greatly appreciated.
    An experimenter measures 9934 counts during one hour from a radioactive sample. From this number the counting rate of the sample can be estimated with a standard deviation of most nearly
    (A) 100
    (B) 200
    (C) 300
    (D) 400
    (E) 500
    the answer is (A) 100, could anyone please help me out understand this problem :confused:

    thanks so much!
  2. jcsd
  3. Aug 31, 2004 #2
    the standard deviation in counting experiments is sqrt(N). In this case N is roughly 10000.

    p.s. i have no clue why.
  4. Aug 31, 2004 #3


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    Staff Emeritus
    Science Advisor
    Gold Member

    This one is more or less just remembernig the rules for estimating error.

    As you probably guessed, the rule for estimating the error of a count is the square root of the count.

    If you're comfortable with statistics, you could probably derive the mean and variance of the "counting" distribution (quotes because I don't remember the right name), use the observed value as an estimate of the mean, and solve for the variance.
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