On 17/01/2013 07:00, r-sig-mixed-models-request at r-project.org wrote:
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https://stat.ethz.ch/mailman/listinfo/r-sig-mixed-models or, via email, send a message with subject or body 'help' to r-sig-mixed-models-request at r-project.org You can reach the person managing the list at r-sig-mixed-models-owner at r-project.org When replying, please edit your Subject line so it is more specific than "Re: Contents of R-sig-mixed-models digest..." Today's Topics: 1. Offset vs fixed factor in a mixed poisson model (v_coudrain at voila.fr) ---------------------------------------------------------------------- Message: 1 Date: Thu, 17 Jan 2013 11:08:51 +0100 (CET) From: v_coudrain at voila.fr To: r-sig-mixed-models at r-project.org Subject: [R-sig-ME] Offset vs fixed factor in a mixed poisson model Message-ID: <436652575.252611358417331707.JavaMail.www at wwinf7130> Content-Type: text/plain; charset=UTF-8 Dear subscribers,
Valerie,
I am tested the effect of a factor on a count variable using a poisson mixed model. I know that my response variable is linearly influenced by an other variable so
Keep in mind that you are using an exponential relationship in a GLM...at least if you use the log link.
that I would like to remove the effect of this second variable to see the true effect of my factor. In an anova, it is usual to enter the covariable first in the model and use a sequential test (type I SS). However I am a bit confused how to control for this covariable in my mixed-poisson model. If I just give the covariable as an additional fixed variable, my factor is highly significant. If I put it instead as an offset, the factor is not significant at all. I think that it is better to use offset, but I must
If you use a covariate as an offset then you essentially saying: double the value of the variable used for the offset, double the numbers (strictly speaking: the expected value). Quite often sampling effort is used as an offset as it is not really interesting to model a cause-effect relationship between sampling effort and your response. If you have a model with: glm(y ~ x, family = poisson) glm(y ~ x + offset(z), family = poisson) and x is significant in the first model...but not in the second, then either the offset explains most variation, or x and the offset are highly correlated? Plot x versus z...and plot x versus log(z)... Alain
admit that the underlying "theory" is not clear for me. I was also wondering if we can specify multiple offsets and if there was some "rule of thumb" in the maximal number that can be included. Thank you very much. Best, Valerie
___________________________________________________________ Envie de changer de frigo ou de gazini?re ? Les soldes ?lectrom?nager sont sur Voila.fr http://shopping.voila.fr/vitrine/electromenager ------------------------------ _______________________________________________ R-sig-mixed-models mailing list R-sig-mixed-models at r-project.org https://stat.ethz.ch/mailman/listinfo/r-sig-mixed-models End of R-sig-mixed-models Digest, Vol 73, Issue 21 **************************************************
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