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

[Rich Shepard]

On Wed, 16 May 2012, Highland Statistics Ltd wrote:

> There is a limit to what can be done with stats. Do you really want to
> analyse data that has been collected by different folks, labs, etc.? Any
> change over time may actually reflect a lab effect.

Alain,

   Within the ivory tower of academic ecology such considerations have
weight. The world of the environmental regulator is a totally different
situation. Yes, laboratories and methods have changed over the decades, and
detection limits have lowered in some cases. But, these are the data used to
issue or renew operating permits and businesses need those permits to
survive and prosper.

> If you have a detection limit then you may want to consider models with a 
> truncated distribution..:-)

   Yep. This is one option I'm going to look at very closely. I started
reading the pdf accompanying the NADA package and have ordered the book on
which it's based.

Regards,

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: 503-667-8863