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Homework Help: Statistics: paried observations

  1. Jul 6, 2011 #1
    1. The problem statement, all variables and given/known data

    To test the efficacy of a new cholesterol-lowering medication, 10 people are selected at random. Each has their LDL levels measured (shown below as Before), then take the medicine for 10 weeks, and then has their LDL levels measured again (After).

    Before After
    195 175
    131 129
    141 105
    182 184
    173 138
    123 120
    147 115
    156 144
    125 87
    161 137

    Give a 95.8% confidence interval for meanB - meanA , the difference between LDL levels before and after taking the medication.

    2. Relevant equations

    d(bar) - t(alpha/2)(Sd/sqrt(n)) < meanDifference < d(bar) + t(alpha/2)(Sd/sqrt(n))

    3. The attempt at a solution

    I found all the values using excel:

    d(bar) = 20
    t(alpha/2) = 2.368676
    Sd = 190.36
    n = 10 (given)

    and my answer was (-122.586 < meanDifference < 162.586)

    but this is incorrect
  2. jcsd
  3. Jul 7, 2011 #2

    Ray Vickson

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    Homework Helper

    I don't know what your Sd is, but if it is the sample standard deviation of the difference, then it is wrong. I get sample mean difference = m = 20, sample standard dev. of diff. = s = 15.3695 (your Sd?). This gives a confidence interval very different from yours.

  4. Jul 7, 2011 #3
    yes it was the standard deviation of the means and I don't get why it would be wrong, I used excel to calculate it..

    Anyways the correct interval was (8.5,31.5)
  5. Jul 7, 2011 #4

    Ray Vickson

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    Science Advisor
    Homework Helper

    I first used Maple to do the calculation. Then, to see if Excel was giving you the trouble, I re-did the analysis using Excel. It agreed with the Maple calculation and disagreed with yours. I suggest you search for a typo or something similar.
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