R help - Apr 2014 - linear mixed model for non-normal negative and continous data

Dear all

 

I try to fit a linear mixed model to my data. In short, my dependent variable
reflects changes of the bone level (Knmn, in mm), thus this variable is
continous and provides negative values. I have two different groups (factor
Group) that were measured 3 times each (thus repeated measures, factor
Timepoint). I used the following model:

 

mod_Knmn<-lmer(Knmn~Group*Timepoint+(1|VPnr),data=data)

 

When performing a qq-plot my residuals are clearly deviant from the norm
(long-tailed). Due to negative values I cannot perform classical transformation
methods (e.g. log transformation). How could I proccede with this data. Is there
a possibility to use a generalized linear model?

 

Thanks and all the best

Caroline
 		 	   		  
	[[alternative HTML version deleted]]

Dear Caroline,

Check the homogeneity of the variances. If they are inhomogeneous, you can add a
variance function to deal with it. However, you will need to switch to the lme()
from the nlme package.

Best regards,

Thierry

PS R-Sig-mixed-models is a better list for this kind of questions.

ir. Thierry Onkelinx
Instituut voor natuur- en bosonderzoek / Research Institute for Nature and
Forest
team Biometrie & Kwaliteitszorg / team Biometrics & Quality Assurance
Kliniekstraat 25
1070 Anderlecht
Belgium
+ 32 2 525 02 51
+ 32 54 43 61 85
Thierry.Onkelinx at inbo.be
www.inbo.be

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asking him to perform a post-mortem examination: he may be able to say what the
experiment died of.
~ Sir Ronald Aylmer Fisher

The plural of anecdote is not data.
~ Roger Brinner

The combination of some data and an aching desire for an answer does not ensure
that a reasonable answer can be extracted from a given body of data.
~ John Tukey

-----Oorspronkelijk bericht-----
Van: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org]
Namens Caroline Lustenberger
Verzonden: maandag 28 april 2014 12:04
Aan: r-help at r-project.org
Onderwerp: [R] linear mixed model for non-normal negative and continous data

Dear all



I try to fit a linear mixed model to my data. In short, my dependent variable
reflects changes of the bone level (Knmn, in mm), thus this variable is
continous and provides negative values. I have two different groups (factor
Group) that were measured 3 times each (thus repeated measures, factor
Timepoint). I used the following model:



mod_Knmn<-lmer(Knmn~Group*Timepoint+(1|VPnr),data=data)



When performing a qq-plot my residuals are clearly deviant from the norm
(long-tailed). Due to negative values I cannot perform classical transformation
methods (e.g. log transformation). How could I proccede with this data. Is there
a possibility to use a generalized linear model?



Thanks and all the best

Caroline

        [[alternative HTML version deleted]]

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