On 16/05/2012 06:00, r-sig-ecology-request at r-project.org wrote:
Please respond to
Rich Shepard
<rshepard at appl-ec
osys.com>
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.
Rich
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. If you have a detection limit then you may want to consider models with a truncated distribution..:-) Alain
Dr. Alain F. Zuur First author of: 1. Analysing Ecological Data (2007). Zuur, AF, Ieno, EN and Smith, GM. Springer. 680 p. URL: www.springer.com/0-387-45967-7 2. Mixed effects models and extensions in ecology with R. (2009). Zuur, AF, Ieno, EN, Walker, N, Saveliev, AA, and Smith, GM. Springer. http://www.springer.com/life+sci/ecology/book/978-0-387-87457-9 3. A Beginner's Guide to R (2009). Zuur, AF, Ieno, EN, Meesters, EHWG. Springer http://www.springer.com/statistics/computational/book/978-0-387-93836-3 4. Zero Inflated Models and Generalized Linear Mixed Models with R. (2012) Zuur, Saveliev, Ieno. http://www.highstat.com/book4.htm Other books: http://www.highstat.com/books.htm Statistical consultancy, courses, data analysis and software Highland Statistics Ltd. 6 Laverock road UK - AB41 6FN Newburgh Tel: 0044 1358 788177 Email: highstat at highstat.com URL: www.highstat.com URL: www.brodgar.com