# Standard Deviation question?

• Oct 7th 2007, 10:15 PM
packcrush
Standard Deviation question?
I have the following problem that I'm having great difficulty solving:

One group of students (sample size a) scored an average of b on an exam. The standard deviation of these results was c. Another group of students (sample size x) scored an average of y in the same exam, with a standard deviation of z. What is the mean and the standard deviation of the two groups of students as a whole?

I know that the weighted mean is given by (a.b+x.y)/(a+x); but I have no idea how to get the weighted standard deviation. Any ideas?
• Oct 8th 2007, 08:45 PM
CaptainBlack
Quote:

Originally Posted by packcrush
I have the following problem that I'm having great difficulty solving:

One group of students (sample size a) scored an average of b on an exam. The standard deviation of these results was c. Another group of students (sample size x) scored an average of y in the same exam, with a standard deviation of z. What is the mean and the standard deviation of the two groups of students as a whole?

I know that the weighted mean is given by (a.b+x.y)/(a+x); but I have no idea how to get the weighted standard deviation. Any ideas?

I think you need the pooled variance formula.

RonL