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Pre-Post Test Improvement

  1. Mar 28, 2009 #1
    Hello There,

    I'm after a little advice; I have data collected during experimentation that was designed to determine the improvement of two groups of subjects between a pre and a post-test. The results are as follows:
    Pre & Post test numbers represent the number of errors made
    Group 1:
    Code (Text):

    0   0   0
    1   0   1
    5   4   1
    12  0   12
    2   0   2
    7   6   1
    0   0   0
    13  0   13
    Group 2:
    Code (Text):

    0   1   -1
    2   0   2
    1   0   1
    0   0   0
    0   11  -11
    5   23  -18
    I'm applying the t-test to determine if there is a statistical difference between the means(of the improvement columns).

    My question is really simple(hopefully)... in group 2, should I be recording improvement as a negative number? or simply zero?

    Using zero will not give me a significant statistical difference between the means of group 1 and group 2... using the negative numbers does.

    Also, if I'm keeping the negative improvement numbers, is it okay to represent improvement as a negative percentage?

    I've not done much with statistics, so would appreciate your input.

  2. jcsd
  3. Mar 29, 2009 #2


    User Avatar
    Homework Helper

    Since you are subtracting in the order Pre-Post, a negative difference indicates a higher value in the post data - that's what you are hoping to find, if I understand you correctly.

    Note that you are free to subtract in either order: if the negative differences are giving you a conceptual problem, subtract in the other order.

    I would be more concerned about doing t-tests with such small samples.
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