[R-sig-eco] Continuous (Non-Count) Skewed Data With Many Zeros
Tom_Philippi at nps.gov
Tom_Philippi at nps.gov
Tue May 15 20:32:59 CEST 2012
Rich--
Are those true zeros, or below detection limit values? 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...
Tom 2
-------------------------------------------
Tom Philippi, Ph.D.
Quantitative Ecologist
Inventory and Monitoring Program
National Park Service
Tom_philippi at NPS.gov
http://science.nature.nps.gov/im/monitor
-------------------------------------------
Rich Shepard
<rshepard at appl-ec
osys.com> To
Sent by: r-sig-ecology at r-project.org
r-sig-ecology-bou cc
nces at r-project.or
g Subject
[R-sig-eco] Continuous (Non-Count)
Skewed Data With Many Zeros
05/15/2012 11:15
AM MST
Please respond to
Rich Shepard
<rshepard at appl-ec
osys.com>
The water chemistry data of metal concentrations are not normally
distributed (based on Q-Q plots) and are not improved by transformation
(log10, sqrt, cubic root). For the 30 metal species the percentage of zeros
ranges from none (10 metals) to 48.6; average 5.6. Most metals are at very
low concentrations with infrequent spikes which might be very high.
Those with fewer zeros are not a concern, but I'd like your thoughts on
1)
at what percentage do the number of zeros become a concern and 2) how to
characterize and model these data.
Rich
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