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-boun...@r-project.org [mailto:r-sig-ecology-boun...@r-project.org] On Behalf Of shylock holmes Sent: Friday, September 27, 2013 12:32 PM To: r-sig-ecology@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 [[alternative HTML version deleted]] _______________________________________________ R-sig-ecology mailing list R-sig-ecology@r-project.org https://stat.ethz.ch/mailman/listinfo/r-sig-ecology This electronic message contains information generated by the USDA solely for the intended recipients. Any unauthorized interception of this message or the use or disclosure of the information it contains may violate the law and subject the violator to civil or criminal penalties. If you believe you have received this message in error, please notify the sender and delete the email immediately. _______________________________________________ R-sig-ecology mailing list R-sig-ecology@r-project.org https://stat.ethz.ch/mailman/listinfo/r-sig-ecology
