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List:       r-sig-mixed-models
Subject:    [R-sig-ME] Correct method?
From:       Chris Mcowen <cm744 () st-andrews ! ac ! uk>
Date:       2011-02-21 9:31:47
Message-ID: 19ED9DC8-382E-4277-8719-C96F811F209E () st-andrews ! ac ! uk
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Dear List,

I am relatively new to mixed models and models in general so i apologise in advance

I have a binary response, 0,1 and two random effects  A & B. I then have a range of \
predictor values which can be put into groups.

Biological -C,D,E,F,G,H,I,J,K
Human - L,M,N,O,P
Environment - Q,R,S,T,U
Spatial - V,W,X

I am interested in asking two questions - Which predictors in each group are the most \
important

Biological_Model <- lmer(yesno~1+(1|A/B)+C,D,E,F,G,H,I,J,K, family=binomial)
I have constructed a series of pre-dfinied models and compared AIC rather than using \
stepwise regression. I have done this for all the groups.

I am now interested, and this is where i am struggling, to investigate which of the \
four groups and combinations therein are most important in determining the response \
and how much do they each contribute?

I have tried this, based on the results from above

Biological <-cbind(C,F,G,J)
Human <-cbind(M,N)
Environment <- cbind(R,S,T)
Spatial <-cbind(X)


I have then run through the combinations of the groups of predictors i.e

Human_Environment_model <- lrm(yesno~Human+Environment)
Spatial_Biological_model <- lrm(yesno~Spatial+Biological)

I have compared the r-squared values and not surprisingly the greater the number of \
packages the better the fit. To counter this i also calculated the AIC value of the \
various models, and this pretty much agrees.

The idea was to look at how much influence these packages have - can you look at the \
difference in R-squared values i.e if i have a Biological model with a r-squared \
value of 0.2 then i add human impact and it goes up to 0.35, can i say the addition \
of human impact added 0.15 or is this not statistically sensible? Or would it be more \
sensible to use the AIC values which indicate Human & Environment describe the \
relationship best?

Thanks

Chris

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