Rich:  I repeat - check out quantile regression in the quantreg package. 
If you have censored data (below detection limits), there are censored 
versions of quantile regression that can be used, crg() function has 3 
alternative estimators.

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:
Rich Shepard <rshepard@appl-ecosys.com>
To:
r-sig-ecology@r-project.org
Date:
05/15/2012 03:52 PM
Subject:
Re: [R-sig-eco] Continuous (Non-Count) Skewed Data With Many Zeros
Sent by:
r-sig-ecology-bounces@r-project.org



On Tue, 15 May 2012, Tom_Philippi@nps.gov wrote:

> Are those true zeros, or below detection limit values?

Tom,

   Recorded as a combination of both. Data were collected over ~31 years 
by
different folks, analyzed by different labs, and stored in spreadsheets
(gak!) with no standardized formatting. It's probably reasonable to assume
that any entry of zero is really below the detection limit; if the 
chemical
was actually not present it would still be below the detecion limit.

> My only experience with water chemistry data had non-detects, not zeros,
> and I had to use the NADA package. On the bright side, I didn't have to
> worry about lots of zeros...

   I'll go find the NADA package.

Thanks,

Rich

-- 
Richard B. Shepard, Ph.D.          |   Integrity - Credibility - 
Innovation
Applied Ecosystem Services, Inc.   |    Helping Ensure Our Clients' 
Futures
<http://www.appl-ecosys.com>     Voice: 503-667-4517      Fax: 
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