[R-sig-eco] data normality

[Bob O'Hara]

On 05/06/2012 10:54 AM, Yong Shen wrote:
> Dear all,
>    I have two questions about data normality.
>    I used stepwise multiple regression to determine which variables contributed to tree growth, and want to built a model to explain tree growth. Sample size is about 50 tree species, I think it is not a large sample size, and some variables are not normal distribution.
> 1. Do I have to transform them to normal distributions before I perform multiple regression?
No. The only area where a Normal assumption comes in is that the 
residuals are normally distributed. So you can happily fit the model 
without worrying about normality until after you've got the model.
> 2. Two variables can not transform to normal distributions although I used some methods (e.g log, sqrt, boxcoxfit), what should I do for the two variables?

Leave them as they are.

Advice that makes life simpler - always the best sort.

Bob

-- 
Bob O'Hara

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