Thanks for your interest and prompt answer! What I try to estimate is the correlation of one bird species counts with a set of environmental parameters. The count data are zero-inflated and overdispersed. I am modeling with hurdle-negative binomial-mixed effects. The results are very difficult to interpret and it get easier dropping out 3 outliers. But I do not know if I should do this.. Thanks! Marta
> Subject: Re: [R] Outliers and overdispersion > From: szehn...@uni-bonn.de > Date: Tue, 13 Aug 2013 17:41:10 +0200 > CC: r-help@r-project.org > To: lomasv...@hotmail.com > > I do not know what you are exactly estimating, but if it is about count > models and the model fit gets better when you drop the outliers, it does not > say, that the model is now more correct. It just says, if the data were > without the outliers, this model would fit good. > > Overdispersion in count data is sometimes a cue, that you have a mixture > distribution as the generating process - for example instead of one, K > different (sub)species of birds which were aggregated in the count data. In > this case a mixture (negative binomial)- distribution with K components could > fit the data better. > > > Best > > Simon > > On Aug 13, 2013, at 5:28 PM, Marta Lomas <lomasv...@hotmail.com> wrote: > > > > > > > > > Hi again, > > > > I have a question on some outliers that I have in my response variable > > (wich are bird counts). At the beginning I did not drop them > > out because they are part of the normal counts and I considered them > > "ecologically" correct. > > > > However, I > > tried some of the same models without ouliers and the AICs are thus better. > > I > > also have nice significances this way... > > > > > > So would you say that, even though the outliers are right > > observations and taking into consideration that already the negative > > binomial > > distribution that I am using is accounting for the some of the > > overdispersion due to the outliers, it is > > better to drop them out as the models fit better this way? > > > > > > Thanks for your patience! > > > > :) > > > > > > > > > > > > > > [[alternative HTML version deleted]] > > > > ______________________________________________ > > R-help@r-project.org mailing list > > https://stat.ethz.ch/mailman/listinfo/r-help > > PLEASE do read the posting guide http://www.R-project.org/posting-guide.html > > and provide commented, minimal, self-contained, reproducible code. > [[alternative HTML version deleted]] ______________________________________________ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
