Hi Jonathan, I don't know if I got the point, but for thresholds assessments, give a look at segmented package.
bests milton brazil=toronto On Tue, Jun 16, 2009 at 5:23 PM, Jonathan Greenberg <greenb...@ucdavis.edu>wrote: > Rers: > > I have some ecological data (stream velocity vs. % cover of submerged > weeds) that shows strong evidence of a thresholding step-function, e.g. > below some velocity, % cover ranges from 0% to 100% (with no apparent > relationship to velocity within this range of velocities), but above a > certain "threshold" velocity, the % cover does not appear to exceed, say, > 10%. There are good mechanistic reasons for believing there is a step > function there, so I'm trying to determine the velocity position and > significance of this threshold. I came up with the following approach, but > I was hoping to find out if there is a more "standardized" way of doing > this: > > 1) Give small velocity steps ranging from 0 to max velocity, classify all > samples (% cover vs. velocity) into "above threshold" and "below threshold". > 2) Perform a t-test on these two groups, store the p-value to an array, > building up a database of p-value vs. velocity threshold. > 3) Determine the minimum p-value from the previous step, which should be > the velocity threshold -- the p-value at this threshold is the significance > of the threshold. > > Does this make sense? Is there a better way of doing this? When I ran > this on the data, you see a nice, nearly parabolic relationship around what > the threshold appears to be of p-value vs. threshold. > > --j > > -- > > Jonathan A. Greenberg, PhD > Postdoctoral Scholar > Center for Spatial Technologies and Remote Sensing (CSTARS) > University of California, Davis > One Shields Avenue > The Barn, Room 250N > Davis, CA 95616 > Cell: 415-794-5043 > AIM: jgrn307, MSN: jgrn...@hotmail.com, Gchat: jgrn307 > > ______________________________________________ > R-help@r-project.org mailing list > https://stat.ethz.ch/mailman/listinfo/r-help > PLEASE do read the posting guide > http://www.R-project.org/posting-guide.html<http://www.r-project.org/posting-guide.html> > and provide commented, minimal, self-contained, reproducible code. > [[alternative HTML version deleted]] ______________________________________________ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
