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I'm reading an article where the AOR was computed after adjusting for confounders; when I calculated the OR it was lower than the AOR presented in the article. Why did the AOR increase? I thought it would be lower after adjustment

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When you adjust anything can happen. For an example consider the following very simple data-set.

    r   n  size treat
1  81  87 small     a
2 234 270 small     b
3 192 263 large     a
4  55  80 large     b

taken from http://www.bmj.com/content/292/6524/879, an article comparing two treatment methods on two different sizes of renal stones where there were $r$ successes out of $n$ people treated.

The log odds ratio for small compared to large unadjusted is 1.070 and for treatment b compared to a is 0.2899. If you adjust each for the other we get 1.2606 and -0.3572 respectively. So small is now further from unity after adjustment and treat is larger in magnitude but with a different sign.

For further details of this example see this Wikipedia article

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