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Thread: Paired T-Test Debate

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    Paired T-Test Debate




    Hey guys! I was having a little debate with a doctor on her study about which statistical test to use. I was always under the assumption that a paired t-test was for "pre-post" tests until this whole idea of "matched controls" rocked my world. This was her study design...

    Study Design
    28 kids who had a disorder
    28 matched controls
    Outcome Variable: Stomach Fat Percent (continuous)

    She wanted to see if there was a difference between the Disorder group and the Control group. I hadalways thought to run an Independent Samples T-Test until the doctor told me otherwise. Can anyone give me insight on what test to run and the theory behind it? Thank you!

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    Re: Paired T-Test Debate

    Well if there was matching done then it sounds like a paired t-test would be the best option.
    I don't have emotions and sometimes that makes me very sad.

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    Re: Paired T-Test Debate

    I agree with Dason.
    “In God we trust. All others must bring data.”
    ~W. Edwards Deming

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    Re: Paired T-Test Debate

    I agree with Jake.

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    Re: Paired T-Test Debate

    I agree with GretaGarbo and of course Jake and Dason....

    If the elements are logically linked to each other you do a paired t-test. A problem I have with matching is that you can never be truly sure they share all values in common except the experiment - as you can for example when simply looking at the same person at two points in time. But that is a design not statistical issue.
    "Very few theories have been abandoned because they were found to be invalid on the basis of empirical evidence...." Spanos, 1995

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    Re: Paired T-Test Debate

    But you can't be sure of that even in the case you mention. The point isn't to get *perfect* matching. It's to reduce variation by providing a pairing that makes sense. You do your best to eliminate the other factors that you want to control for by matching. You don't need to do a *perfect* job of this to see a reduction in the variance.
    I don't have emotions and sometimes that makes me very sad.

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    Re: Paired T-Test Debate

    Statistically I am sure that is true. But from a design perspective if the individuals are not identical except on the experiment you can never be certain that the experiment caused the result. The say you can with random assignment, but even ignoring this is rarely done in practice, it never seemed logical to be that if the groups end up being systematically different on some factor after random assignment that this has no effect simply because you assigned randomly.

    Regardless it is a design issue not statistical.
    "Very few theories have been abandoned because they were found to be invalid on the basis of empirical evidence...." Spanos, 1995

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    Re: Paired T-Test Debate

    With matching, you have no independent observations (I suppose)
    in each groups. So a dependent samples test should be appropriate.
    On the other hand, I have often seen such designs analysed with
    independent sample tests. Admittedly, in my first published study
    I used an independent samples test for a matched pairs desgin, and
    no reviewer objected. So, is it a matter of different viewpoints,
    or is the independent samples analysis just wrong?

    With kind regards

    K.

    Extravert

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    Re: Paired T-Test Debate

    The independent and dependent test calculate variation differently I thought since in the latter case the observations are related to each other, but in the former they are not. This makes one test invalid for the other.
    "Very few theories have been abandoned because they were found to be invalid on the basis of empirical evidence...." Spanos, 1995

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    Re: Paired T-Test Debate

    Try a scatter plot of the "matched" pairs. If you see a relationship a paired approach should be appropriate. However, if you see a shotgun pattern use a 2-sample t.

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    Re: Paired T-Test Debate

    Here is my data: Should I use a 2-sample t-test?

    Name:  shotgun.jpg
Views: 118
Size:  10.1 KB
    I don't have emotions and sometimes that makes me very sad.

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    Re: Paired T-Test Debate

    Is this real or manufactured data?

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    Re: Paired T-Test Debate

    I don't know but it has that shotgun pattern you were talking about
    I don't have emotions and sometimes that makes me very sad.

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    Re: Paired T-Test Debate

    Quote Originally Posted by Dason View Post
    I don't know but it has that shotgun pattern you were talking about
    HAhahaha
    not the "shotgun pattern" we're used to

    (Edit b/c I thought that was the OP's data at first, then realized it wasn't)
    The mathematical explanation of a statistical procedure is really just pseudo-code, which we can make operational by translating it into real computer code. --B. Klemens

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    Re: Paired T-Test Debate


    Nope - just me making a lame joke.
    I don't have emotions and sometimes that makes me very sad.

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