My statistical knowledge is very limited.
I have two groups (patients and controls) and a measured concentration value for each indivdual. Not normally distributed: Mann-Whitney
How do I correct for gender in Mann-Whitney? (I want to know if the difference between patients and controls is due to difference in gender distribution, since males have higher values than females).
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2 Answers
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You cannot correct for gender in Mann-Whitney. Mann-Whitney is a location test for two groups, and that's all.
There are at least two options here: 1) Stratify by gender. That is, analyze the men and women separately.
2) Do some sort of regression, perhaps OLS or, given your use of Mann-Whitney, perhaps quantile regression, with "concentration" as the dependent variable and two independent variables: Gender and group (patient vs. control)
answered Mar 15, 2013 at 10:32
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$\begingroup$ Is there any other non-parametric test that allows me to correct for gender? $\endgroup$– CamillaCommented Mar 15, 2013 at 11:12
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$\begingroup$ "Nonparametric" covers a wide range of things, including types of regression. I don't know of anything like the Mann Whitney U that corrects for another factor - it's almost part of the definition: Mann Whitney U is simple precisely because it doesn't allow such things. $\endgroup$ Commented Mar 15, 2013 at 11:49
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1$\begingroup$ Can I perform a Kruskal-Wallis with 4 groups? (male+patient, male+control, female+patient, female+control) $\endgroup$– CamillaCommented Mar 15, 2013 at 14:17
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The closest I think you could get with some existing test, is if your data came from a randomized complete block design, but if I recall correctly, this requires people to be randomly assigned to the genders (blocks): this may be rather hard :). If you could, however, you could use a Mack-Skillings test.
You state that your statistical knowledge is limited, so the only true solution I have to offer may be somewhat out of your league: you can build a probabilistic index model (see http://onlinelibrary.wiley.com/doi/10.1111/j.1467-9868.2011.01020.x/full ). These can be used to extend the classical distributionfree tests (like Mann-Whitney) in a similar fashion as you would do through a GLM (a colleague of mine should have a paper published on that any day now), but it would lead me too far to try and explain everything around that theory here.
So: unless you're willing to go the PIM-route, I don't think there is a solution, currently, for distributionfree tests.
answered Mar 15, 2013 at 10:34
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$\begingroup$ Thank you for your answer, unfortunately it sounds to complicated for me. Can I perform a mack-Skillings test in SPSS? $\endgroup$– CamillaCommented Mar 15, 2013 at 11:33