[R-sig-Geo] geoR questions

Rubén Roa Ureta rroa at udec.cl
Fri Jun 12 21:46:27 CEST 2009

María Eugenia Utgés wrote:
> ---------------------------- Original Message ----------------------------
> Subject: Presentation and a "few" Questions (geoR)
> From:    María Eugenia Utgés <meutges at ege.fcen.uba.ar>
> Date:    Wed, June 10, 2009 5:21 pm
> To:      r-sig-geo at stat.math.ethz.ch
> --------------------------------------------------------------------------
>  Hello Everyone
> I am making my doctorate in biology at the University of Buenos Aires,
> Argentina.
> I am working on survival and dispersal of the south american fruit fly
> (Diptera: Tephritidae), a fruit pest mainly of citrics.
> To do so I have field dispersal data (with its x,y coordinates) and have
> already perform some spatial analysis with geoR.
> My specific questions are:
> 1- Data must show normality?  What to do if even transformed data does not
> adjust?
geoR has the option to carry out a Box-Cox transform of your data so it 
will be as close to normal as data allows.
The transformation is controlled by the lambda parameter and this can be 
one of the arguments of the likelihood function.
> 2- How do I interpret the cross-validation results?
What was your question?
> 3- How do I know if I need to take into account anisotropy?
First, do the anysotropic variogram plot, function variog4.

> 4- When to end the iteration of likfit? till stable values of estimated
> parameters?
It will end by itself, last time I checked.
> 5- Is it recommended to present the image of the kriging AND the one
> representing variances? how do I generate the latter?
Krigins is only necessary if you want to make prediction, but it doesn't 
hurt to present your predictions.
What do you mean by variances? Do you mean the kriging variances?
If so, do the normal kirging and ask geoR to predict variances instead 
of predictions in the function krige.conv
> 6- Do the kriging image values have a unit, or are they arbitrary?
Predictions are back-transformed from Box-Cox so they have your original 
> 7- Finally, if you had to present the results (in a paper for example)
> what results/data/graphics would you choose?
You can check the many papers on the subject. One of them is mine, 
Roa-Ureta and Niklitschek, 2007,
ICES Journal of Marine Science 64: 1723–1734.


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