[R-sig-eco] Continuous (Non-Count) Skewed Data With Many Zeros

[Rich Shepard]

   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