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Repeated measure comparisons: should the identity be a random or a fixed variable?

Billy 
Hello everybody!

I'm relatively new at the mixed-models world and I'm facing a
theoretical/philosophical problem.
Let's go to my data collection.

I wanna compare the number of eggs laid by females (different
individuals or the same, I have no idea) at the time 1 and at the time
2 in the same location. Therefore, I have repeated measures by
location and wanna compare time 1 versus time two. Given I have count
data, to minimize the overdispersion I have considered the Poisson
distribution for the errors.
Furthermore, I have collected this data throughout one year and I'm
also interested in temporal variation among months.

model0 <- glmer ( y ~ 1 + (1|location) + (1|month), family="poisson")
model1 <- glmer ( y ~ x + (1|location) + (1|month), family="poisson")

where y = number of eggs laid,
          x = factor concerning the first or the second oviposition
          location = factor concerning the exactly position in the
space (just an identity of the oviposition site and responsible for
the repeated comparison)
          month = factor concerning the month when I've collected the data

Is that right? If I wanna repeated comparison regarding specific
identity of oviposition sites, should this factor (location) be a
random variable?

Furthermore, in both examples above, I'm just considering a temporal
variation (among months) as random a effect. But I'm also interested
if there are significant seasonal variation in the comparison (the
difference could be higher during warm season or not even existent
during cold season). Then:

model2 <- glmer ( y ~ x + month + (1|location), family="poisson")
model3 <- glmer ( y ~ x * month + (1|location), family="poisson")

Is that right too?
Finally, I'll use a model selection approach to compare the different
models and rank the most likely one to reproduce the data observed in
the nature.
Thanks to everyone

Thread (3 messages)

Billy Repeated measure comparisons: should the identity be a random or a fixed variable? Nov 26 Jarrod Hadfield Repeated measure comparisons: should the identity be a random or a fixed variable? Nov 27 Billy Repeated measure comparisons: should the identity be a random or a fixed variable? Nov 30