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Calculating the variance with a TWIST

  1. Dec 2, 2008 #1
    Calculating the variance with a TWIST!!!

    Hello!! I am trying to understand this problem!! its just on my practice questions and Ihave the answer, I CAN NOT understand how to set it up!! PLEASE someon ehelpo!!!!!

    THis is it :)

    A student wants to calculate the variance of a set of 10 scores. But he doesnt have hte raw scores, but only has the deviation of each raw score from the mean. Worse yet he only has 9 of tese deviation scores...

    compute the variance for him

    -5, +11, -4, -2, +7, -8, -6, +1, -3
  2. jcsd
  3. Dec 3, 2008 #2
    Re: Calculating the variance with a TWIST!!!

    Well, the mean of the deviations from the mean must be zero, so to find the 10th score deviation, pick the one that makes the mean of the 10 values equal to 0.

    Then just compute the variance of those 10; remember, variance is unchanged by equal shifts of all the values
  4. Dec 4, 2008 #3


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    Re: Calculating the variance with a TWIST!!!

    Your first step in calculating the variance would be to take the distance from each element to the mean, right? So it shouldn't be a problem that this is done for you. The last missing distance will of course be the negative of the sum of the other distances, since the overall sum needs to be 0 (otherwise, the mean wouldn't be the mean!).
  5. Dec 4, 2008 #4


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    Re: Calculating the variance with a TWIST!!!

    Once you have the tenth deviation, the sample variance is the sum of the squares of these deviations divided by n-1.
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