On 12 Nov 2018, at 12:26, roimaor at post.tau.ac.il wrote:
Thanks for your response Walid.
However, it doesn't really answer my questions, so I'll try to clarify those.
An intercept estimate in my model makes no biological sense because all species have both diet and habitat. This is why I chose to suppress the intercept.
In the context of categorical predictors, suppressing the intercept means that an arbitrarily-chosen category is taken as baseline, and the model estimates the difference (in the response variable) between this baseline and all other categories.
When dealing with two categorical traits as predictors, each data point is a combination of the effects of both traits, i.e. it is a combination of one category from each trait.
If this is indeed the case, the baseline value must include one category of each predictor in the model, otherwise their effects would be confounded.
For example, in my model I would expect one diet category AND one habitat category, say, 'herbivorous' and 'aquatic', to be baseline/intercept (un-estimated).
Instead, it seems that only 'herbivorous' is absorbed in the intercept, while all habitat classes have posterior estimates.
My questions are:
1- why does no habitat category get absorbed in the intercept? and
2- what can I do to fix that?
If it's at all useful, here is an example:
## data format
unique(Tdata$Habitat)
[1] TE AR AQ ST SA
Levels: AQ AR SA ST TE
unique(Tdata$Diet)
[1] Herb Omni Faun
Levels: Faun Herb Omni
ModTHRE <- MCMCglmm(Response ~ -1 + Habitat + Diet,
prior = list(R = list(V = 1, nu = 0.002)),
ginverse = list(Binomial=INphylo$Ainv),
burnin = 25000, nitt = 225000, thin = 200,
family = "threshold",
data = Tdata,
verbose = FALSE)
## model output
summary(ModTHRE)
Iterations = 25001:224801
Thinning interval = 200
Sample size = 1000
DIC: 2321.861
R-structure: ~units
post.mean l-95% CI u-95% CI eff.samp
units 1 0.9973 1.003 1000
Location effects: Response ~ -1 + ForagingHab + Troph_Lev
post.mean l-95% CI u-95% CI eff.samp pMCMC
HabitatST -0.07612 -0.42629 0.33746 1000.0 0.694
HabitatTE -0.36682 -0.52533 -0.21118 1000.0 <0.001 ***
HabitatSA -0.04691 -0.25769 0.19636 1000.0 0.724
HabitatAR -0.07302 -0.29900 0.16681 883.3 0.548
HabitatAQ -0.47948 -0.82598 -0.15793 1000.0 0.002 **
DietHerb 0.30708 0.11255 0.48545 1000.0 0.002 **
DietOmni 0.05263 -0.12176 0.26105 1000.0 0.604
---
Signif. codes: 0 ?***? 0.001 ?**? 0.01 ?*? 0.05 ?.? 0.1 ? ? 1
Cutpoints:
post.mean l-95% CI u-95% CI eff.samp
cutpoint.traitActivity.1 0.4911 0.436 0.5495 877.6
Any thoughts, ideas of what's going on here, or corrections of my mis-perceptions, are all very welcome.
Many thanks,
Roi
Quoting Walid Mawass <walidmawass10 at gmail.com>:
Hello,
by adding -1 into your model's formula, you are explicitly calling for no
intercept term to be included in the estimates. If you do want an intercept
term which would include a baseline level for each of your categorical
variables then the syntax should be:
MCMCglmm(Activity ~1 + Habitat + Diet + log(Mass) + Max.Temp... (explicit
call for intercept term)
or
MCMCglmm(Activity ~ Habitat + Diet + log(Mass) + Max.Temp... (implicit call
for intercept term)
Hope this helps
--
Walid Mawass
Ph.D. candidate in Cellular and Molecular Biology
Population Genetics Laboratory
University of Qu?bec at Trois-Rivi?res
3351, boul. des Forges, C.P. 500
Trois-Rivi?res (Qu?bec) G9A 5H7
Telephone: 819-376-5011 poste 3384
On Wed, Nov 7, 2018 at 12:09 PM <roimaor at post.tau.ac.il> wrote:
Dear list members,
I have a model with categorical response and categorical + continuous
predictors.
My model has two categorical predictors: "diet" (3 levels) and
"habitat" (6 levels):
THRE1 <- MCMCglmm(Activity ~ -1 + Habitat + Diet + log(Mass) + Max.Temp,
prior = list(R = list(V = 1, fix = 1)),
ginverse = list(Binomial=INphylo$Ainv),
family = "threshold",
data = Tdata)
If I understand correctly, in this configuration the algorithm
shouldn't return estimated values for the effect of each level of a
categorical predictor, instead, it returns a contrast between that
level and another level which was arbitrarily chosen as the base
level. Each species (data point) has a value for each of these traits,
so I would expect them to be estimated independently, meaning that one
level of each predictor should be the 'baseline' and absorbed into the
global intercept. In that case I expect 2 contrasts to be returned for
diet categories and 5 contrasts for habitat.
However, I get 2 estimates (presumably contrasts) for diet categories,
and 6 for habitat categories, i.e., no habitat category was designated
as baseline, which makes me question whether the estimates are
contrasts or actual effect sizes.
My questions:
- Is the algorithm pooling all the predictor categories as if they
were a single trait with 8 levels?
- If the habitat effect estimates are contrasts - what are they compared
to?
- If they are effect sizes - what did I do to not get the contrasts as
I expected?
Any help would be much appreciated!
Thanks,
--
Roi Maor
PhD candidate
School of Zoology, Tel Aviv University
Centre for Biodiversity and Environment Research, UCL