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Thread: How do I show something is an 'independent variable'?

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    How do I show something is an 'independent variable'?




    Please help. I am totally confused about how to show that something is an independent risk factor.

    I.e. in a cohort study:

    Group A (n=1000) - exposure to X - outcome exposed 70/1000 demonstrate Z

    Group B (n = 1000) - no exposure -- outcome control 23/1000 demonstrate Z

    I know that I can calculate the Risk Ratio to compare Exposure X to control, to see how much more likely the risk is for Group A given exposure X.

    However, to complicate matters there is an exposure (Y) that moves across both groups. I.e.:

    of Group A 750 also exposed to Y -- 60 demonstrate Z

    Group B 300 also exposed to Y -- 16 demonstrate Z

    How do I show that Y is an 'independent risk factor' for Z.

    I.e. is there a formula I use to show that Y is an independent risk factor for Z?

    Thanks!

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    Re: How do I show something is an 'independent variable'?

    Something is an independent variable because you believe it is driving change not being driven by some other factor. I am not sure that is what you mean by an independent risk factor however.

    Independence and dependence is driven by theory not a formula you can run (although you can test if there is a statistical relationship between X and Y you can't show empirically X-> Y rather than Y-> X or X and Y effect each other at the same time. This is normally done through theory not data analysis).
    "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: How do I show something is an 'independent risk factor'.


    Quote Originally Posted by noetsi View Post
    Something is an independent variable because you believe it is driving change not being driven by some other factor. I am not sure that is what you mean by an independent risk factor however.
    Thank you! Yes - you are right, was not looking for 'independent variable' -- I was looking for how to work out whether something was an 'independent risk factor'. I realised that my question was related to whether the second criteria (Y) may be confounding the outcome... I.e. To establish whether Y may be confounding the results need to show:
    1) that Y is an independent risk factor; 2) that Y is not an intervening variable and 3) that Y is associated with the study factor.

    I couldn't work out how to show (1).. But then realised that what I had to do was look only at the non-exposed people. I calculated the RR for them in relation to Y. (16/300 -exposed)/(7/700-) = 5.3... So in the control group people exposed to Y have a 8.8 greater risk than those not exposed to Y of developing Z. I would argue that this shows that Y is an independent risk factor (as there is no exposure to X; exposure to Y; and an increased risk of developing Z).

    (2) and (3) are satisfied too.
    Last edited by RoseMarie; 04-03-2013 at 11:59 AM.

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