Hello,?
I have a question regarding the function Betareg. I am a bit new to this package, and maybe I did not understand all the theory behind...
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My data are proportions (%) of a?contaminant (DecaBDE) in blood of birds (n=78) which?were GPS-tagged so that we could see what kind of habitat they visited.?We are investigating the effects of habitat use on the proportions of that contaminant.
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The full model is as follow: %contaminant = % time spent in agriculture + %
time in urban + % time in St-Lawrence River + having visited a landfill (yes/no). ?
Coding:
Deca.sumBDE <- betareg (DecaJb.sumBDEs ~ Agri.outCol24 + AllLawren.outCol24 + LandfiWstwater.YesNo
+ UrbanCov1.outCol24, data = mydata).
Because I am not yet completely?familiar with all the theory underlying beta regressions, I?followed?one of the examples in the package and?created a basic model?i.e., no second part?specified?to model the precision (as you can see from the coding).?
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Somehow?all my models produced with betareg (including the intercept only) end up with a negative intercept - but I have no negative data. I was also expecting that the intercept of the null model would be the average of my dependant variable (as in a regular linear regression), but it is not....
Any suggesions ? Could this be?happening because I did not fit a two parts model ??Or did I simply misunderstand
how to interpret and use beta regressions?
Thank you for your time,
Marie
PhD candidate
Canada???????