Gre Problem # 40 counting rate

[quantumworld]

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

thanks so much!

 

[suffian]

the standard deviation in counting experiments is sqrt(N). In this case N is roughly 10000.

p.s. i have no clue why.

 

[Hurkyl]

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.