[R-sig-ME] Adjusting for random recording intervals in glmer/poisson

Dieter Menne dieter.menne at menne-biomed.de
Thu Jul 5 08:16:33 CEST 2012

Joshua Wiley wrote:

> What is the significance of the recording interval varying?  If the
> issue is that with a longer recording time, there are more
> opportunities for events to occur, then what about treating duration as
> an exposure and including it in the offset?  Essentially you model rate
> then rather than counts.

Good to hear that you suggest it to put it into the offset; I wanted to do this, but was not sure what exactly to put into the offset term. Duration or log(duration)?


Apologies: I forgot to attach the simulated sample data in the original message

nsubj = 10
nvisit = 5
d = data.frame(
  subj = as.factor(1:nsubj),
  duration = runif(nsubj*nvisit,30,60),# in minutes
  predictor = rnorm(nsubj*nvisit,50,10))
d$nevent = with(d,rpois(nsubj*nvisit,predictor*duration/500))

# Proposed solution by university statistician: 
# use only the data from the first 30 minutes (not shown here) and do
glmer(nevent~predictor + (1|subj),data=d, family=poisson)
# Result is not correct, because truncated data not used

# Proposed by Joshua
glmer(nevent~predictor+offset(log(duration)) + (1|subj), data=d, family=poisson)

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