# Gre Problem # 40

1. Aug 31, 2004

### 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

thanks so much!

2. Aug 31, 2004

### suffian

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

p.s. i have no clue why.

3. Aug 31, 2004

### Hurkyl

Staff Emeritus
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.