# Thread: considering covariates in a non parametric test

1. ## considering covariates in a non parametric test

Hello, I have the following problem:
I have to analyze two variables, both continuous (they are results of body tests).
Since I have cases only and I tested the variables to check the distribution (which was not normal) I ran a spearman correlation. Actually this was the approach chosen even by former studies studying similar things.

my tutor asked me to adjust this for covariates like sex, disease duration and age...

But this changes the approach completely. I don't know any non parametric test that allows to do that for continuous variables. Any suggestion?

2. ## Re: considering covariates in a non parametric test

Run a multiple linear regression and check the distribution of
the residuals. The dependent variable doesn't need to be
normally distributed, only the residuals. And if the sample size
is large enough, then departures form normality usually don't
matter much. Maybe you want to make a little search for linear
regression assumptions.

With kind regards

K.

3. ## The Following User Says Thank You to Karabiner For This Useful Post:

Rerun (04-12-2014)

4. ## Re: considering covariates in a non parametric test

I checked the distribution of the independent variable using the Shapiro-wilk/francia and sktest and it is not normally distributed!
That is why I did not go ahead with multiple linear regression. Is my assumption wrong?

thank you very much

5. ## Re: considering covariates in a non parametric test

I checked the distribution of the independent variable using the Shapiro-wilk/francia and sktest and it is not normally distributed!
There's no need for an independent variable in regression to be
normally distributed, only the model residuals. And if the sample size
is large enough, then departures form normality usually don't
matter much. Maybe you want to make a little search for linear
regression assumptions. http://pareonline.net/pdf/v18n11.pdf
for example.

With kind regards

K.

6. ## The Following User Says Thank You to Karabiner For This Useful Post:

Rerun (04-12-2014)

7. ## Re: considering covariates in a non parametric test

Another viable alternative you could consider is a Bootstrap Regression technique. Of course, you would need the appropriate software (S-Plus, R) to perform this technique.

http://cran.r-project.org/doc/contri...tstrapping.pdf

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