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Math Help - SD and Mean

  1. #1
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    SD and Mean

    ok i got the mean which is 300 and the SD is 70. What is the first quartile score? how can i get this number?
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  2. #2
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    Quote Originally Posted by makaveli89 View Post
    ok i got the mean which is 300 and the SD is 70. What is the first quartile score? how can i get this number?
    You can't get the quartiles from this information, at least not without additional assumptions about the distribution.

    The first quartile q_1 is such that:

    p(x < q_1)=1/4

    RonL
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  3. #3
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    i dont get it
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    Quote Originally Posted by makaveli89 View Post
    i dont get it
    If you have calculated the mean and standard deviation from a sample then you need to calculate the 1-st quartile from the sample, you can't get it from the maen and standard deviation.

    If the mean and standard deviation are from a theoretical distribution you need to know what that distribution is to calculate the 1-st quartile.

    In other words post the entire question.

    RonL
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    it is normally distributed with a mean of 300 and a standard deviation of 70.
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    Quote Originally Posted by makaveli89 View Post
    it is normally distributed with a mean of 300 and a standard deviation of 70.
    Let Q denote the cumulative distribution of the standard normal distribution. The the 1-st Quartile of the standard normal is:

    q_1=Q(0.25)

    and that of a normal distribution of mean \mu and sd \sigma is:

    q_1(\mu,\sigma) = \mu + \sigma q_1

    RonL
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  7. #7
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    Quote Originally Posted by makaveli89 View Post
    it is normally distributed with a mean of 300 and a standard deviation of 70.
    It's funny what a difference two additional words can make. Next time make sure all the information is included.
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