[R-sig-ME] Repeated measure comparisons: should the identity be a random or a fixed variable?
billy.requena at gmail.com
Tue Nov 30 21:26:15 CET 2010
Thanks for the reply.
I followed your suggestion to treat months as continuous variable and
the model selection result was totally different. In the first
analyses, model 2 (considering an additive effect of x and month) was
the most likely model, but after implemented your suggestion, the best
model was model 1 (considering only the effect of x, with month as a
random variable). However, I think in such case (following your
suggestion) there is an assumption that the third month, for example,
is in a higher level than the first or second month, right? Maybe
that's not the case in nature and my goal is exactly investigate if
the relationship between y and x could change from one month to
another, not necessarily always increasing or always decreasing. In
fact, the first sampled month is August and the last one is July of
the next year. Assuming a seasonal variation, maybe a should use a
circular statistics approach to deal with this case, I don't know.
Furthermore, I don't know if I really understand your other suggestion.
Thanks again and sorry for some misunderstandings.
PhD student - Laboratory of Arthropod Behavior and Evolution
Universidade de São Paulo
a/c Glauco Machado
Departamento de Ecologia - IBUSP
Rua do Matão - Travessa 14 no 321 Cidade Universitária, São Paulo - SP, Brasil
Phone number: 55 11 3091-7488
On Sat, Nov 27, 2010 at 2:26 AM, Jarrod Hadfield <j.hadfield at ed.ac.uk> wrote:
> Hi Billy,
> I think your models look reasonable, although in models 2 and 3 you may want
> to treat month as a continuous variable in the fixed part of the model.
> Also, most count data are overdispersed with respect to the poisson and so a
> model that does not account for this will be anti-conservative in terms of
> standard errors etc. One way to deal with this is to fit an additional
> random effect at the level of each observation:
> and fit (1|resid) in the model formula.
> Quoting Billy <billy.requena at gmail.com>:
>> 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
>> Gustavo Requena
>> PhD student - Laboratory of Arthropod Behavior and Evolution
>> Universidade de São Paulo
>> Correspondence adress:
>> a/c Glauco Machado
>> Departamento de Ecologia - IBUSP
>> Rua do Matão - Travessa 14 no 321 Cidade Universitária, São Paulo - SP,
>> CEP 05508-900
>> Phone number: 55 11 3091-7488
>> R-sig-mixed-models at r-project.org mailing list
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