[R-sig-eco] generating equation (linear or non-linear) from scatter plot data

[Baldwin, Jim -FS]

Do you have multiple measurements on individuals?  If so, have you accounted for the "repeated measures"?  If there are no multiple measurements on individuals, then maybe what explains a decrease in body size is a selection bias:  only smaller individuals survive in later time periods resulting in a smaller mean body size in later times.  So you would be seeing a change in mean size rather than individuals shrinking.

Jim Baldwin


-----Original Message-----
From: r-sig-ecology-bounces at r-project.org [mailto:r-sig-ecology-bounces at r-project.org] On Behalf Of shylock holmes
Sent: Friday, September 27, 2013 12:32 PM
To: r-sig-ecology at r-project.org
Subject: [R-sig-eco] generating equation (linear or non-linear) from scatter plot data

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)......





thank you for any suggestions on this......m

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