[R-sig-ME] Log-normal MCMCglmm

[Paul Johnson]

Hi Dani,

Can you explain with a formula what you mean by the difference between a model with log-normal errors and a model with normal errors where the response is log-transformed? As the errors are log-normal, i.e. strictly positive, they presumably can’t be additive. Are they multiplicative? If they are then logging the predictor will result in an additive model with normal errors.

E.g. a multiplicative model with lognormal errors:
Y = Yhat * exp(epsilon), where epsilon ~ exp(N(0, sigma^2))

Log it:
log(Y) = log(Yhat) + log(epsilon)
which is an additive model with normal errors with the response log-transformed.

Best wishes,
Paul

On 28 Jul 2015, at 09:47, Daniel Sol <dsolrueda at gmail.com> wrote:

> Hi Jörg,
> 
> Thanks a lot for the suggestion. I actually have tried to use a Poisson
> error, but it looks like my data best fit a log-normal distribution.
> 
> Best,
> 
> Dani
> 
> 2015-07-28 10:40 GMT+02:00 Jörg Albrecht <albrechj at staff.uni-marburg.de>:
> 
>> Hi Dani,
>> 
>> you could try specifying
>> 
>> family = "poisson".
>> 
>> Best,
>> 
>> Jörg
>> 
>> —
>> Jörg Albrecht, PhD
>> Postdoctoral researcher
>> Institute of Nature Conservation
>> Polish Academy of Sciences
>> Mickiewicza 33
>> 31-120 Krakow, Poland
>> www.carpathianbear.pl
>> www.globeproject.pl
>> www.iop.krakow.pl
>> 
>> Am 25.07.2015 um 11:31 schrieb Daniel Sol <dsolrueda at gmail.com>:
>> 
>> Hi everybody,
>> 
>> I have trouble finding how to implement a MCMCglmm with log-normal error. I
>> know some people just log-transform the response variable, but this is not
>> the same.
>> 
>> Many thanks in advance,
>> 
>> Dani
>> 
>> --
>> Daniel Sol
>> CREAF (Centre for Ecological Research and Forestry Applications)
>> CSIC (Spanish National Research Council)
>> Bellaterra, Catalonia E-08193, Spain
>> TEL: +34 93-5814678
>> FAX: +34 93-5814151
>> E-MAIL: d.sol at creaf.uab.es
>> Webpage: http://dsolrueda.wix.com/sol-group
>> 
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>> 
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>> 
>> 
>> 
> 
> 
> -- 
> Daniel Sol
> CREAF (Centre for Ecological Research and Forestry Applications)
> CSIC (Spanish National Research Council)
> Bellaterra, Catalonia E-08193, Spain
> TEL: +34 93-5814678
> FAX: +34 93-5814151
> E-MAIL: d.sol at creaf.uab.es
> Webpage: http://dsolrueda.wix.com/sol-group
> 
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> 
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