[R-sig-ME] Unclear output from MCMCglmm with categorical predictors

[r***@post.tau.ac.il]

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,

[Walid Mawass]

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 Québec at Trois-Rivières
3351, boul. des Forges, C.P. 500
Trois-Rivières (Québec) G9A 5H7
Telephone: [phone] poste 3384

Post by r***@post.tau.ac.il
Dear list members,
I have a model with categorical response and categorical + continuous
predictors.
My model has two categorical predictors: "diet" (3 levels) and
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.
- 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
_______________________________________________
https://stat.ethz.ch/mailman/listinfo/r-sig-mixed-models

[[alternative HTML version deleted]]

[r***@post.tau.ac.il]

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

Post by Walid Mawass
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
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 Québec at Trois-Rivières
3351, boul. des Forges, C.P. 500
Trois-Rivières (Québec) G9A 5H7
Telephone: [phone] poste 3384

Post by r***@post.tau.ac.il
Dear list members,
I have a model with categorical response and categorical + continuous
predictors.
My model has two categorical predictors: "diet" (3 levels) and
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.
- 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
_______________________________________________
https://stat.ethz.ch/mailman/listinfo/r-sig-mixed-models

[HADFIELD Jarrod]

Hi,

The habitat coefficients are the estimates when (in the example) diet is
Faun. The two diet coefficients are the differences between Herb and
Faun and Herb and Omni. As the previous poster said, removing the -1
will give you an intercept and habitat coefficients that are differences
between the absorbed (base-line) habitat and the remaining habitats.

Cheers,

Jarrod

Post by r***@post.tau.ac.il
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.
1- why does no habitat category get absorbed in the intercept? and
2- what can I do to fix that?
## 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
                         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

Post by Walid Mawass
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
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 Québec at Trois-Rivières
3351, boul. des Forges, C.P. 500
Trois-Rivières (Québec) G9A 5H7
Telephone: [phone] poste 3384

Post by r***@post.tau.ac.il
Dear list members,
I have a model with categorical response and categorical + continuous
predictors.
My model has two categorical predictors: "diet" (3 levels) and
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.
- 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
_______________________________________________
https://stat.ethz.ch/mailman/listinfo/r-sig-mixed-models

_______________________________________________
https://stat.ethz.ch/mailman/listinfo/r-sig-mixed-models

[HADFIELD Jarrod]

Hi,

Sorry - I also noted some errors in your example, and the original code.

1/ In the example you do not fix the residual variance (to one) which you should do because it is not identifiable in threshold models.

2/ You have what seems to be a phylogenetic correlation structure passed to ginverse, but you don’t associate it with a random term.

Cheers,

Jarrod

Post by r***@post.tau.ac.il
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.
1- why does no habitat category get absorbed in the intercept? and
2- what can I do to fix that?
## 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
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

Post by Walid Mawass
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
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 Québec at Trois-Rivières
3351, boul. des Forges, C.P. 500
Trois-Rivières (Québec) G9A 5H7
Telephone: [phone] poste 3384

Post by r***@post.tau.ac.il
Dear list members,
I have a model with categorical response and categorical + continuous
predictors.
My model has two categorical predictors: "diet" (3 levels) and
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.
- 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
_______________________________________________
https://stat.ethz.ch/mailman/listinfo/r-sig-mixed-models

_______________________________________________
https://stat.ethz.ch/mailman/listinfo/r-sig-mixed-models

[r***@post.tau.ac.il]

Hi Jarrod,
Thanks so much for the helpful comments.
I was just about to check why models with and without the ginverse
argument were giving similar estimates when a strong phylogenetic
signal is expected... Much appreciated!
And thanks again Walid- I now understand what you meant in your answer.

All the best,
Roi

Post by HADFIELD Jarrod
Hi,
Sorry - I also noted some errors in your example, and the original code.
1/ In the example you do not fix the residual variance (to one)
which you should do because it is not identifiable in threshold
models.
2/ You have what seems to be a phylogenetic correlation structure
passed to ginverse, but you don’t associate it with a random term.
Cheers,
Jarrod

Post by r***@post.tau.ac.il
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.
1- why does no habitat category get absorbed in the intercept? and
2- what can I do to fix that?
## 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
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

Post by Walid Mawass
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
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 Québec at Trois-Rivières
3351, boul. des Forges, C.P. 500
Trois-Rivières (Québec) G9A 5H7
Telephone: [phone] poste 3384

Post by r***@post.tau.ac.il
Dear list members,
I have a model with categorical response and categorical + continuous
predictors.
My model has two categorical predictors: "diet" (3 levels) and
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.
- 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
_______________________________________________
https://stat.ethz.ch/mailman/listinfo/r-sig-mixed-models

_______________________________________________
https://stat.ethz.ch/mailman/listinfo/r-sig-mixed-models

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