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Math Help - random variable - mean, variance, standard deviation

  1. #1
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    random variable - mean, variance, standard deviation

    I'm given a table
    -1 0 1
    .31 .38 .31

    the question is asking me to find mean of X, variance of X, and standard deviation of X. The answers are
    mean = 0
    variance = .62
    standard deviation = .787

    I've been in class everyday and don't think the teach even went over this... I'm looking through the text book in chapter we are studying and found formulas which aren't giving me the correct answers...

    I'm not even exactly sure how to get the mean. originally I tried adding up the probabilities (.31 + .38 + .31)/3 = .33 but that wasn't correct. I was supposed to do that for -1, 0, 1...

    from my understanding the variance is supposed to be
    sum of "value x prob" = -.31 + 0 + .31 = 0 but the answer is .62... why? I've done variance before with that formula, but it's different this time?

    I've tried standard deviation using the .62 variance given to me. (-1 - .62)^2 x (.31) + (0 - .62)^2 x (.38) + (1 - .62)^2 x (.31) ...it doesn't = .787 and I've tried experimenting a few different possibilities and still can't reach that number.

    I'm lost...
    Last edited by jepherz; May 6th 2012 at 01:24 PM.
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  2. #2
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    Re: random variable - mean, variance, standard deviation

    Dont worry, you just have your formulae mixed up.

    "sum of value x prob" is the mean. You have correctly evaluated this as zero, which matches the answer in the question.

    Write down the formula you have been given for the variance, and your attempt to evaluate it, and we'll go from there
    Thanks from jepherz
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  3. #3
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    Re: random variable - mean, variance, standard deviation

    wow.... I was going back and forth with this for about an hour and my standard deviation kept giving me .62 which is the answer for variance and I just sqrt that to get 787............ you are right, I got the formulas kinda mixed up. thanks for the help! now I need to go back to figure out another problem I'm stuck on >.>
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