[R] weights in quantile regression

Chris Wilcox c.wilcox at uq.edu.au
Thu Jul 18 20:54:38 CEST 2002

I am just starting to use R, converting over from SAS.  I want to estimate
the upper quartile in a scatterplot of demographic rates against rainfall in
a small mammal population.  The data looks like this:

|*                  * <- point in question
|*  *
|*  *
|*  *   *
|*  *   *   *   *   *

Based on a paper by Scharf et al. in Ecology (vol 79, p448) I am trying to
use the qreg package in R to estimate the upper quartile in the data.  My
problem is that I have relatively few data points (~40) which are mostly
distributed at the low end of the x axis, and an outlier in the y direction
at the high end of the x axis.

I would like to find a way either by iteratively weighting the observations
or by bootstrapping the regression to quantify the relationship in the mass
of the data.  I looked in the manual for qreg, and while it allows for
weights, there is no guidance for routines to calculate them.  I also
checked the help archives and on the web, and while I found a bit on the
web, I didnt see much that was specific to my problem.  It seems like this
would be a general issue when working with naturally occurring phenomena
(like rainfall) as there will often be few data points at the extremes.

Any suggestions would be most welcome.



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