Interpreting Brms Hurdle Result

Interfaces brms
1

Short summary of the problem
I am currently studying the effects of exposure on horn length in female bovids; however, some species have females that lack horns completely. I need to run a phylogenetic analyses that takes in account the zero inflation in my continuous horn length data. I used the function “brm” from the package “brms” where I could specify the “family” function to use a hurdle model, and it successfully ran. However, I am not sure what are the differences between the hurdle model options (i.e. hurdle_gamma vs hurdle_poisson).
In the output below, I used “hurdle_gamma”. If that happens to be a correct way to analyze this data, I wanted to see if it be possible to help me understand the results correctly. It seems there is a significant effect of expo (exposure score) on horn length, but I am not sure if hurdle_gamma was the correct family choice.



fit1 <- brm(horn~expo + (1|gr(phylo, cov = Horn)) + (1|Binomial), data = HornGamma, family = hurdle_gamma(), data2 = list(Horn = Horn), control = list(adapt_delta = 0.99), iter = 4000)

summary(fit1)
 Family: hurdle_gamma
  Links: mu = log; shape = identity; hu = identity
Formula: horn ~ expo + (1 | gr(phylo, cov = Horn)) + (1 | Binomial)
   Data: HornGamma (Number of observations: 89)
  Draws: 4 chains, each with iter = 4000; warmup = 2000; thin = 1;
         total post-warmup draws = 8000

Group-Level Effects:
~Binomial (Number of levels: 89)
              Estimate Est.Error l-95% CI u-95% CI Rhat Bulk_ESS Tail_ESS
sd(Intercept)     0.09      0.05     0.01     0.18 1.00      663     1501

~phylo (Number of levels: 89)
              Estimate Est.Error l-95% CI u-95% CI Rhat Bulk_ESS Tail_ESS
sd(Intercept)     0.05      0.02     0.02     0.09 1.01     1285     1594

Population-Level Effects:
          Estimate Est.Error l-95% CI u-95% CI Rhat Bulk_ESS Tail_ESS
Intercept     0.68      0.15     0.40     0.98 1.00     3559     4086
expo          0.18      0.06     0.07     0.30 1.00     4116     5245

Family Specific Parameters:
      Estimate Est.Error l-95% CI u-95% CI Rhat Bulk_ESS Tail_ESS
shape    73.94     59.24    23.83   234.87 1.01      496      803
hu        0.35      0.05     0.26     0.45 1.00    15527     5044

Draws were sampled using sampling(NUTS). For each parameter, Bulk_ESS
and Tail_ESS are effective sample size measures, and Rhat is the potential
scale reduction factor on split chains (at convergence, Rhat = 1).
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2

Would you be able to post the full model call? I am guessing it’s length ~ … ? Do you have a histogram plot of horn length that you can share as well?

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