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I'm working on research into the vitamin D levels of professional and amateur soccer players, and relationship with factors such as skin color, vitamin D-intake etc.

I don't know which test to use. I'll give you an idea of the situation:

  • I have a continuous variable: the measured vitamin D levels
  • A categorial variable: the skin color (either black or white )

I've read about the One-way ANOVA, but it requires three options for the categorical variable. I used the ANOVA test, the p-value was 0.005. Is it possible to use this test with just two options for the categorical variable? Or is there another test I could use?

And if it's possible to use the ANOVA, does the value F mean anything, or just the p-value?

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    $\begingroup$ Hi @Joost, welcome to the site. If you have only two groups (black and white), you could do a simple two-sample t-test to compare the mean vitamin D levels between black and white skin color. By the way: what software do you use? Stata? SPSS? R? $\endgroup$ – COOLSerdash Jun 8 '13 at 19:55
  • $\begingroup$ I am actually looking for the correlation/association between the vitmamin D status and the skin color, sorry if my question wasnt clear enough. Any ideas on which test to use in this case? $\endgroup$ – Joost Jun 8 '13 at 20:16
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    $\begingroup$ Okay, then you could run a regression model with vitamin D as dependent and the skin color as independent categorical variable. But if you have only those two variables, that's basically the same as a two-sample t-test (because you have only 2 categories). $\endgroup$ – COOLSerdash Jun 8 '13 at 21:00
  • $\begingroup$ If you want a correlation coefficient, you could calculate the Point-biserial correlation coefficient which is just the normal Pearson correlation coefficient where one variable is dichotomous (skin color). So just calculate the normal correlation coefficient between vit. D and skin color. $\endgroup$ – COOLSerdash Jun 8 '13 at 21:15
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Anova with two categories is the same as the two-sample t-test (apart from the standard presentation of the results). You can check yourself that the square of the t-value equals the F-value. The same model can also be presented as a linear regression with skin-color as a categorical predictor, coded via dummys.

You should also visualize your data, in your case a parallel boxplot could be a good start.

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