R help - Dec 2006 - Offset for Poisson GLM

Dear R-help,
I am trying to fit a Poisson GLM to counts of dolphin sightings. Counts
represent the number of dolphin sightings made per unit effort, that is, per
full hour of on-effort watching. I want to add an offset so that the number
of hours watched is incorporated into the model. But since the number of
observations in a given year, or hour of the day is the desired offset,
rather than a column in my data frame, I'm not sure how to specify that as
an offset in the model:

glm(counts ~ as.factor(year) + as.factor(month) + as.factor(TOD) +
sightability, data=count.data, family=poisson, offset=log(???))

Any ideas will be much appreciated!

Thanks,
Theoni Photopoulos

-- 
Theoni Photopoulos, MRes Candidate
Gatty Marine Laboratory
School of Biology
University of St Andrews
St Andrews, Scotland
tp14@st-andrews.ac.uk

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