 Hi, I would suggest that you look at zero-inflated models for continuous responses.Good luck,Salvador  Salvador Sánchez-ColónConsultoría Ambiental y EstadísticaMéxico DFMobile: 04455-3200-8559E-mail: salvadorsanchezcolon@prodigy.net.mx
De: r-sig-ecology-bounces@r-project.org
Para: r-sig-ecology@r-project.org
Cc: 
Fecha: Tue, 15 May 2012 11:15:39 -0700 (PDT)
Asunto: [R-sig-eco] Continuous (Non-Count) Skewed Data With Many Zeros

>    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> > _______________________________________________> R-sig-ecology mailing list> R-sig-ecology@r-project.org> https://stat.ethz.ch/mailman/listinfo/r-sig-ecology

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