Hi, I am trying to use quantile regression to perform weighted-comparisons of the median across groups. This works most of the time, however I am seeing some odd output in summary(rq()):
Call: rq(formula = sand ~ method, tau = 0.5, data = x, weights = area_fraction) Coefficients: Value Std. Error t value Pr(>|t|) (Intercept) 45.44262 3.64706 12.46007 0.00000 methodmukey-HRU 0.00000 4.67115 0.00000 1.00000 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ When I do not include the weights, I get something a little closer to a weighted comparison of means, along with an error message: Call: rq(formula = sand ~ method, tau = 0.5, data = x) Coefficients: Value Std. Error t value Pr(>|t|) (Intercept) 44.91579 2.46341 18.23318 0.00000 methodmukey-HRU 9.57601 9.29348 1.03040 0.30380 Warning message: In rq.fit.br(x, y, tau = tau, ...) : Solution may be nonunique I have noticed that the error message goes away when specifying method='fn' to rq(). An example is below. Could this have something to do with replication in the data? # example: library(quantreg) # load data x <- read.csv(url('http://169.237.35.250/~dylan/temp/test.csv')) # with weights summary(rq(sand ~ method, data=x, weights=area_fraction, tau=0.5), se='ker') # without weights # note error message summary(rq(sand ~ method, data=x, tau=0.5), se='ker') # without weights, no error message summary(rq(sand ~ method, data=x, tau=0.5, method='fn'), se='ker') -- Dylan Beaudette Soil Resource Laboratory http://casoilresource.lawr.ucdavis.edu/ University of California at Davis 530.754.7341 ______________________________________________ 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.