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Homework Help: Stats 1 question: standard deviation

  1. Nov 13, 2011 #1
    I missed the lesson on this and i can't figure out how to do this question with the data given.

    1. The problem statement, all variables and given/known data

    A class of 20 students takes a test. The score, x , for each student was recorded by the teacher. The results of summarised by:
    ∑(x-10) = 208 and ∑(x-10)² = 2716

    i) calculate the standard deviation of the 20 scores.
    ii)show that ∑x² = 8876
    iii) Two other students took the test later. Their scores were 18 and 16. Find the mean and standard deviation of all 22 scores.

    2. Relevant equations


    3. The attempt at a solution

    I tried i, ∑(x-10)² / 20 =varience = 2716/20
    standard deviation =√(2716/20)= 11.65
    I think this is wrong though because 10 isn't the mean.
  2. jcsd
  3. Nov 13, 2011 #2

    Ray Vickson

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    Science Advisor
    Homework Helper

    So, you need to find the mean. Alternatively, you can just look at y = x-10 instead of x. How are the variance of x and the variance of y related?

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