[Biomod-commits] prevalence and pseudoabsences

Wilfried Thuiller wilfried.thuiller at ujf-grenoble.fr
Fri Apr 22 07:08:44 CEST 2011

Dear Brenna,

Yes and no... 

If you do not ask for pseudo-absence (NbPA=0), there is no weigthing and all your pseudo-absence will be used at once. Prevalence = 0.1
If you add NbPA = 3040 (or more), yes, there is. The prevalence = 0.5

Does it help?

Le 22 avr. 2011 à 00:53, Brenna Forester a écrit :

> Hello,
> I see in the "Presentation Manual for BIOMOD" (page 18) the following statement: "In all procedures, BIOMOD ensures that the prevalence of the original data is conserved in the calibration and evaluation datasets."
> I have 304 presence records and am running my pseudoabsence pulls with 3040 absences (a prevalence of 0.1).  The number of pixels in my study area is 6808.
> From the above quote, I think that BIOMOD is maintaining the original prevalance of 0.1.  Is that correct?  I just want to be sure that there is no weighting of absence records (e.g. weighting to simulate a prevalence of 0.5).
> Thank you,
> Brenna
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Dr. Wilfried Thuiller
Laboratoire d'Ecologie Alpine, UMR CNRS 5553
Université Joseph Fourier
BP53, 38041 Grenoble cedex 9, France
tel: +33 (0)4 76 51 44 97
fax: +33 (0)4 76 51 42 79

Email: wilfried.thuiller at ujf-grenoble.fr
Personal website: http://www.will.chez-alice.fr
Team website: http://www-leca.ujf-grenoble.fr/equipes/emabio.htm

FP6 European MACIS project: http://www.macis-project.net
FP6 European EcoChange project: http://www.ecochange-project.eu

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