[R-sig-ME] Specifying random effects in lme()

[Gavin Simpson]

Dear List,

I am modelling repeated measures data from 11 sites in the UK using
lme() from the nlme package. I have the following call for a random
intercept model with an AR1 error structure:

mod <- lme(doc ~ temp + log.so4 + log.cl, random = ~ 1 | site,
           correlation = corAR1(), data = hydro)

I would like to fit a model that has random effects for log.so4 and
log.cl within site, so that I am fitting a random slop and intercept
model --- we want the effect of log.so4 and log.cl to vary between sites
and to compare the fits of the two models.

If I am following lmer() in lme4 correctly, I believe I could fit this
model as:

hydro.lmer <- lmer(doc ~ temp + log.so4 + log.cl +
                   (log.so4 | site) + (log.cl | site),
                   data = hydro)

but without the AR1 correlation.

I am struggling to get the syntax correct for random in lme() to add
random effects to log.so4 and log.cl. I have only managed to get a model
where one of the coefficients (log.cl) also has a random effect.

I would be most grateful if someone could explain how to fit the model
in lme() by showing me the correct form for the random argument.

Many thanks,

Gav

-- 
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 Gavin Simpson                 [t] +44 (0)20 7679 0522
 ECRC, UCL Geography,          [f] +44 (0)20 7679 0565
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