On 27-Sep-13, at 12:32 PM, shylock holmes wrote:
Hi all - I have been looking on-line for an answer but cannot find
one thus far, and was hoping anyone could be so kind to assist with
some suggestions.
I have biological data of body size (y) over time (x), and after a
preliminary inspection using a scatter plot the data suggest the
body size is declining. I used a linear regression to confirm this,
however, now I would like to examine what sort of underlying
regression equation I can use to describe this decline, i.e., how
do I get the parameters for the equation and for a more fitting
curve (better than the loess)? I could not find anything online on
this so far.
The curve is what I call a "reversed growth curve", and I also
would like to examine at what year the decline is 5% or greater
from the horizontal/asymptote (if that makes sense)......
To get a meaningful answer you will probably have to specify more
clearly the constraints on what you are doing. For example, if you
use a linear regression, *every* year will have the same slope. If I
understand your question, the answer will depend sensitively on the
type of curve that you fit.
This is not really an R question, and you might want to consult a
statistician, or take a look at Ben Bolker's or Frank Harrell's books
on regression modeling.
thank you for any suggestions on this......m
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Don McKenzie, Research Ecologist
Pacific Wildland Fire Sciences Lab
US Forest Service
phone: 206-732-7824
Affiliate Professor
School of Environmental and Forest Sciences
University of Washington
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