Dear all,
 
We are modelling the total numbers of hibarnating bats in a fortress. We
have data of the number of bats per room spanning ten years. The main
problem is that not all rooms were visited each year. The fieldworkers
did not known or find all rooms and some rooms were not allways
accessible.
 
Some of the rooms were not counted in the early years and they contain a
rather high number of bats in the more recent years. So a glm on the
total observed number would be very biased. Therefore we would use a
mixed model on the numbers of bats per room. The model looks like:
glmer(Number ~ Year + (1|Year) + (Year|Room), family = poisson). Year is
the long-term trend. (1|Year) allows for year-to-year variability (due
to weatherconditions) and (Year|Room) allows for a random intercept and
slope per room.
 
Our main question about this model is the interpretation of the
long-term trend (fixed effect of Year). Given the model specification it
is the trend in an 'average' room from the population of rooms. Can we
assume that this trend equals the trend in the total number of bats in
the fortress. That would be the trend in to total observed numbers if we
could have investigated every room in every year.
Or is it better to use the model to simulate the total number of bats
and then model this simulated totals using a simple glm? Repeating the
simulations a large number of times would yield an average and
confidence intervals for the trend.
 
Best regards,
 
Thierry

------------------------------------------------------------------------
----
ir. Thierry Onkelinx
Instituut voor natuur- en bosonderzoek
team Biometrie & Kwaliteitszorg
Gaverstraat 4
9500 Geraardsbergen
Belgium

Research Institute for Nature and Forest
team Biometrics & Quality Assurance
Gaverstraat 4
9500 Geraardsbergen
Belgium

tel. + 32 54/436 185
Thierry.Onkelinx@inbo.be
www.inbo.be

To call in the statistician after the experiment is done may be no more
than asking him to perform a post-mortem examination: he may be able to
say what the experiment died of.
~ Sir Ronald Aylmer Fisher

The plural of anecdote is not data.
~ Roger Brinner

The combination of some data and an aching desire for an answer does not
ensure that a reasonable answer can be extracted from a given body of
data.
~ John Tukey
  



Druk dit bericht a.u.b. niet onnodig af.
Please do not print this message unnecessarily.

Dit bericht en eventuele bijlagen geven enkel de visie van de schrijver weer 
en binden het INBO onder geen enkel beding, zolang dit bericht niet bevestigd is
door een geldig ondertekend document. The views expressed in  this message
and any annex are purely those of the writer and may not be regarded as stating 
an official position of INBO, as long as the message is not confirmed by a duly 
signed document.

	[[alternative HTML version deleted]]


