[R] A question on lmer() function

[Douglas Bates]

On 12/17/06, Guojing Yang <Guojing.Yang at cdu.edu.au> wrote:
> Dear R users,
>      We have encountered a slight problem when using the lmer()
> function:
>
>   1. Data description: 11 locations; Nt: monthly mosquito population
> density from 1994-2005 in each location.
>   2. Question: to examine the degree of spatial heterogeneity in the
> system by testing model support for single versus multiple intercepts
> and slopes for the location effect. We applied the lmer() function and
> introduced location as spatial effect. Four different models were
> generated as following:
> a.      1 intercept & 1 slope :  lmer(r~log(N)+(1|location)), where r <-
> log(Nt/Nt-1) and location is a random effect.
> b.      Multi-intercept & 1 slope: lmer(r~location+log(N)+(1|location))
> c.      1 slope & multi-intercept: ??????
> d.      Multi-intercept & multi-slope:
> lmer(r~location*log(N)+(1|location))

I'm not exactly sure what you want to do here but you definitely
should not have location as a fixed-effects term and a random effects
term of the form (1|location).  Such terms would be confounded.

I think the models you want to fit would all have an (implicit)
intercept and a log(N) term in the fixed effects.  They would differ
in terms of whether you have a random effect for the intercept by
location and/or a random effect for the slope with respect to log(N)
by location.  Four variants are

#  random effects for intercept only
lmer(r ~ log(N) + (1|location))

# random effects for slope only
lmer(r ~ log(N) + (0+log(N)|location))

# possibly correlated random effects for intercept and slope
lmer(r ~ log(N) + (log(N)|location))

# independent random effects for intercept and slope
lmer(r ~ log(N) + (1|location) + (0+log(N)|location))

> As you can see, we have had trouble defining model c. Does anyone know
> how to code this using lmer? We really appreciate any response.
> Guojing
>
>
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