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Math Help - Standard Deviation question?

  1. #1
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    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?
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  2. #2
    Grand Panjandrum
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    Quote Originally Posted by packcrush View Post
    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
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