[R] regression fit

David Winsemius dwinsemius at comcast.net
Fri Nov 25 22:03:58 CET 2011

On Nov 25, 2011, at 2:56 PM, magushi wrote:

> Hi every one.
> I want to analyze my data. The  y (density) is very skewed to the  
> right.
> this is my tentative fit<-density~factor(Site)+factor(Species)+ depth.
> which model to apply between glm and lm, for sure I can not use  
> lm( This is
> non linear data)

Until you have looked more carefully at your tentative fit you are  
jumping to unsupported assumptions. It is a common misconception that  
a skewed distribution of the dependent variable represents a problem  
for linear methods. It may be true but you will not know until you  
look at the distribution of the residuals of a linear fit. So far we  
see not evidence that you have done so. If you showed us plots of  
density ~ depth within categories of Site and Species then there might  
be a basis for discussion of alternative links or transformations.

This wouldn't be homework by any chance, would it?

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David Winsemius, MD
West Hartford, CT

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