Brian
Thanks for the quick response! You have found a solution. I tried this 
and it worked well. For my current purpose it is perfectly fine to use 
jitter and the result is just as useful. For the one problem subset (of 
160 subsets) the actual regression won't be super useful, but at least 
it won't crash the rest of the process to run all subsets. Those -1 
values are actually log-transformed non-detects, and I am just running a 
visualization routine to get a series of panel scatters for a first look 
at the data.
Thanks for the help...
Steve

===============================================
Steven R. Corsi        Phone: (608) 821-3835
Research Hydrologist   email: srcorsi@usgs.gov
U.S. Geological Survey
Wisconsin Water Science Center
8505 Research Way
Middleton, WI 53562
===============================================


On 7/21/2011 5:37 PM, Brian S Cade wrote:
> Steve:  I would guess that the problem relates to the large number of 
> tied values of -1 in your dependent y variable.  You could randomly 
> jitter these y = -1 by  adding a random uniform number between, say, [ 
> -0.01, 0.01] and see if the rq() converges to a solution.   Then you 
> would know that was the numeric computing issue.   Then the question 
> would be what to do next?  Seems like a funny data problem with a 
> point mass of responses at -1.  Perhaps only higher quantiles, say 
> 0.80, are going to give usable estimates.  Perhaps the jittered 
> responses can yield a reasonably interpreted estimate.
>
> Brian
>
> Brian S. Cade, PhD
>
> U. S. Geological Survey
> Fort Collins Science Center
> 2150 Centre Ave., Bldg. C
> Fort Collins, CO  80526-8818
>
> email:  brian_cade@usgs.gov
> tel:  970 226-9326
>
>
> From: 	Steven R Corsi <srcorsi@usgs.gov>
> To: 	r-help@r-project.org
> Date: 	07/21/2011 04:04 PM
> Subject: 	[R] Quantreg-rq crashing trouble
> Sent by: 	r-help-bounces@r-project.org
>
>
> ------------------------------------------------------------------------
>
>
>
> Hi
>
> I am using the quantreg package for median regression for a large series
> of subsets of data. It works fabulously for all but one subset. When it
> reaches this subset, R takes the command and never responds. I end up
> having to kill R and restart it.
>
> It appears to be something with the particular data subset, but I can't
> pinpoint the problem.
>
> Here are some details
> Operating system: Windows 7
> R version: 2.12.1
>
> Here is the data and the rq command that gives me trouble:
>
> library(quantreg)
>
> x <-
> c(-0.340778085786686,-0.573639751645382,-0.663932762810308,-0.438591328531796,0.302202380883637,-0.675558868120683,-0.764547425063882,-0.751796238115147,-0.481835451050657,-0.588287304540034,-0.622315341312595,-0.542777491991884,-0.552343921339062,-0.587743299883,-0.758233854317935,-0.783134744819092,-0.97774093234124,0.859832969267456,0.69037126308323,0.185409334523753,-0.432951955490942,-0.988120972598647,0.243223425575187)
>
> y<-
>
> c(2.35531739878456,-1,2.26484142532915,-1,2.86895579641295,2.6655997506336,-1,1.33021078457153,-1,-1,-1,1.82633400562222,1.60831204269733,-1,2.45313479655685,-1,-1,-1,-1,-1,-1,-1,-1)
>
>
> fit1 <- rq(y ~ x, tau = .5)
>
>
> Any feedback would be greatly appreciated.
> Thanks
> Steve
>
> -- 
> ===============================================
> Steven R. Corsi        Phone: (608) 821-3835
> Research Hydrologist   email: srcorsi@usgs.gov
> U.S. Geological Survey
> Wisconsin Water Science Center
> 8505 Research Way
> Middleton, WI 53562
>
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