[R-sig-ME] unequal spacing in repeated measures

[Matthew P. Dekar]

I would appreciate any advice in regards to the handling of unequal spacing in repeated measures regression with mixed models.  I sampled crayfish with funnel traps and measured environmental predictor variables monthly for two years in 12 stream pools.  Data are presented in terms of the average number of crayfish per trap per pool (response variable = catch-per-unit-effort = cpue).  However, the sampling interval was not fixed so I created a continuous time variable (day) indicating the number of days elapsed from the first sampling occasion (1,35,55,...,643).  As an example, I modeled cpue in a repeated measures framework with a single predictor variable (stream temperature = temp), pool as the subject/random variable, and day was included using a spatial covariance structure (Exponential = corExp) in lme following:

cpu1<-lme(cpue~temp, random=~1 | pool, data=CPU, method = "ML")
cpu2<- update(cpu1, correlation = corExp(form=~day))

Is this an appropriate usage of spatial covariance structures?  Can the above analysis be replicated in lmer?  Thanks very much for your time.

Matthew Dekar  

Arkansas Cooperative Fish & Wildlife Research Unit
Department of Biological Sciences
University of Arkansas
Fayetteville, AR 72701
(479) 575-6360