On 11-01-13 10:13 AM, Chris Mcowen wrote:
> Hi Ben,
>
> Thanks for the reply:
>
> The Pagel test showed that there is a strong phylogentic signal in my
> data.
>
> I don't believe i used
>> reasonably minor differences in the way that you account for
>> correlation
>
> as i used a method without any account for the correlation i.e GLM,
> which selected the same model set and the same "best" model as GLMM
> the other "phylogentic" methods.
>
> I guess my central point is i spent a while researching methods to
> deal with phylogenetic structure, and there are various schools of
> thought of the best method. Some say using random effects in GLMM
> models is not capable of dealing with phylogentic structure where as
> others suggest the CAIC method is the best as it actually uses the
> tree. So from these points of view there is actually a considerable
> difference in the method used?
>
> If you look at the model set "selected" based on AIC differences by
> each method, they are the same, therefore is any method really better
> for this type of investigation than another?
OK, "relatively minor" was an overstatement (sorry I overlooked the
fact that you used a GLM as well). You can have a strong phylogenetic
signal; that still (as you have demonstrated) doesn't mean that it will
overturn the conclusions based on a non-phylogenetic analysis.
The fact that *for this analysis* all the methods used give the same
answer doesn't invalidate the general point that the different methods
do different things and that some might be better than others.
I guess I'm curious what conclusions you're drawing from this exercise:
* "am I doing something wrong, or missing something obvious"?
- I don't think so.
* "why does everyone make such a fuss about the differences"?
- Because they sometimes (just not in this case) have a big effect
on the conclusions
Ben Bolker
>
> Chris
>
> I think your comparison is really worthwhile, but I don't see why
> this is surprising at all -- perhaps I'm missing something. Given
> that you have reasonably strong effects of your predictors,
> reasonably minor differences in the way that you account for
> correlation won't change the qualitative conclusion
>
> On 13 Jan 2011, at 15:00, Ben Bolker wrote:
>
> On 11-01-13 04:38 AM, Chris Mcowen wrote:
>
>> Dear list,
>
>> I am modelling the effect of various life history traits of
>> species against their extinction rating.
>
>> My data has a phylogentic signal (see table below) so to be
>> statistically correct i worked with phylogentic independent
>> contrasts
>
>
>> From Purvis et al., 2000 "phylogenetic analyses were necessary
>> because of the pseudoreplication and, hence, elevated type I error
>> rates that result from treating species as independent points when
>> relevant variables show a phylogenetic pattern"
>
>> Variable ยป ( Pagel) IUCN extinction risk
>> 0.47 Breeding system
>> 0.99 Endosperm 0.96 Floral symmetry
>> 0.93 Fruit 0.97
>> Pollen dispersal 0.99 Seasonality
>> 0.52 Storage organ 0.85
>> Woodyness 0.61
>
>> There are various ways of doing this one method is to use the
>> package CAIC (generated using compara- tive analysis by
>> independent contrasts) which utilizes the phylogeny to generate
>> the independent contrasts. However as i was using it i found this:
>> from Sodhi et al., 2008
>
>>> It was necessary to decompose the variance across species by
>>> coding the random-effects error structure of the GLMM as a
>>> hierarchical taxonomic (class/order/ family) effect (Blackburn &
>>> Duncan, 2001). We had insufficient replication within genera to
>>> include the genus in the nested random effect. Our method is more
>>> appropriate than the independent-contrasts approach (Purvis et
>>> al., 2000) in situations where a complete phylogeny of the study
>>> taxon is unavailable, when categorical variables are included in
>>> the analysis, and when model selection, rather than hypothesis
>>> testing, is the statistical paradigm being used.
>
>> So i gave this a go - using GLMM and setting the order / family as
>> random effects.
>
>> I then came across mcmcGLMM which allows the use of a phylogentic
>> tree to deal with the phylogenetic structure of the data set.
>
>> So i gave this a go as well!
>
>> Finally to be consistent i ran the model with no phylogenetic
>> control - using GLM.
>
>> My criteria for model selection was that of Burnham and Anderson -
>> using AIC and AICML etc to select the "most likely" set of models.
>
>> Interestingly i found that using all four methods the same model
>> (based on AIC difference) was selected as the most likely?
>> Furthermore, the pattern of AIC differences across the models
>> reflected each other.
>
> I think your comparison is really worthwhile, but I don't see why
> this is surprising at all -- perhaps I'm missing something. Given
> that you have reasonably strong effects of your predictors,
> reasonably minor differences in the way that you account for
> correlation won't change the qualitative conclusions.
>
> Ben Bolker
>
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