Hi Nathan,

I fould this paper by Zuur and co-workers,
2009<http://www3.interscience.wiley.com/journal/122683826/abstract?CRETRY=1&SRETRY=0>,
very insightful. While step-wise modeling is something The R book presents,
it may or may not be the best way to do it. Read the paper and judge for
yourself if it's written to your tune.



On Sat, Feb 6, 2010 at 5:17 PM, Nathan Lemoine <lemoine.nathan@gmail.com>wrote:

> Hi everyone,
>
> I'm trying to fit a multiple regression model and have run into some
> questions regarding the appropriate procedure to use. I am trying to compare
> fish assemblages (species richness, total abundance, etc.) to metrics of
> habitat quality. I swam transects are recorded all fish observed, then I
> measured the structural complexity and live coral cover over each transect.
> I am interested in weighting which of these two metrics has the largest
> influence on structuring fish assemblages.
>
> My strategy was to use a multiple linear regression. Since the data were in
> two different measurement units, I scaled the variables to a mean of 0 and
> std. dev. of 1. This should allow me to compare the sizes of the beta
> coefficients to determine the relative (but not absolute) importance of each
> habitat variable on the fish assemblage, correct?
>
> My model was lm(Species Richness~Complexity+Coral Cover). I had run a full
> model and found no evidence of interactions, so I ran it without the
> interaction present.
>
> It turns out coral cover was not significant in any regression. I have been
> told that the test I used was incorrect and that the appropriate procedure
> is a stepwise regression, which would, undoubtedly, provide me with
> Complexity as a significant variable and remove Coral Cover. This seems to
> me to be the exact same interpretation as the above model. So, since I'm
> very new to all of this, I am wondering how to tell whether one model is
> 'incorrect' or 'inappropriate' given that they yield almost identical
> results? What are the advantages of a stepwise regression over a standard
> multiple regression like I have run?
>
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>



-- 
In God we trust, all others bring data.

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