Ah, okay, my apologies for misunderstanding. To clarify, is your main goal to figure out how to encode a model like the one above or to gain an understanding of how to use Bijectors?

Most users never need to touch Bijectors, as it’s transparently used in the background when needed to transform your model so the parameters are sampled in a latent unconstrained space and then transform them back before returning them to you.

So for your model, you can just do this:

```
using Turing, Distributions, StatsPlots
@model function mn(Y; N = sum(Y), k = length(Y))
θ ~ Dirichlet(k, 1)
Y ~ Multinomial(N, θ)
end
Y = [15, 155, 4, 0, 29, 1]
m = mn(Y)
chn = sample(m, NUTS(500, 0.9), MCMCThreads(), 1000, 4)
```

Note that I’ve switched the sampler to `NUTS`

. This should be the go-to when your parameters are all continuous, since it’s typically more efficient, and when it fails, it can help diagnose problems that cause other samplers to fail silently.

In a first run (using `NUTS()`

), I saw lots of numerical errors (divergences; `sum(chn[:numerical_error])`

). This seems to happen because of the very low counts of `0`

and `1`

; this causes the posterior density to have high curvature, which causes samples to be biased. Increasing to `δ=0.9`

adapts a smaller step size, and we get no numerical error. Here’s the result:

```
julia> chn
Chains MCMC chain (1000×18×4 Array{Float64, 3}):
Iterations = 501:1:1500
Number of chains = 4
Samples per chain = 1000
Wall duration = 4.55 seconds
Compute duration = 4.53 seconds
parameters = θ[1], θ[2], θ[3], θ[4], θ[5], θ[6]
internals = lp, n_steps, is_accept, acceptance_rate, log_density, hamiltonian_energy, hamiltonian_energy_error, max_hamiltonian_energy_error, tree_depth, numerical_error, step_size, nom_step_size
Summary Statistics
parameters mean std naive_se mcse ess rhat ess_per_sec
Symbol Float64 Float64 Float64 Float64 Float64 Float64 Float64
θ[1] 0.0762 0.0185 0.0003 0.0002 5182.5234 1.0000 1145.0560
θ[2] 0.7429 0.0307 0.0005 0.0005 4792.4908 0.9998 1058.8800
θ[3] 0.0240 0.0107 0.0002 0.0002 4574.2447 0.9996 1010.6595
θ[4] 0.0047 0.0045 0.0001 0.0001 5322.6232 1.0001 1176.0104
θ[5] 0.1429 0.0247 0.0004 0.0004 4348.0419 0.9998 960.6809
θ[6] 0.0094 0.0066 0.0001 0.0001 4931.6204 0.9997 1089.6201
Quantiles
parameters 2.5% 25.0% 50.0% 75.0% 97.5%
Symbol Float64 Float64 Float64 Float64 Float64
θ[1] 0.0441 0.0624 0.0752 0.0881 0.1161
θ[2] 0.6818 0.7228 0.7435 0.7642 0.8020
θ[3] 0.0077 0.0163 0.0224 0.0300 0.0490
θ[4] 0.0001 0.0014 0.0033 0.0066 0.0165
θ[5] 0.0972 0.1257 0.1417 0.1595 0.1946
θ[6] 0.0011 0.0046 0.0079 0.0128 0.0262
```

The `rhat`

values close to 1 look good. And note that the `ess`

(effective sample size) values are all greater than the number of requested draws (`4*1000=4_000`

). That indicates antithetic sampling; i.e. NUTS could sample so well that the resulting draws will yield better estimates even than exact (independent) draws from the posterior.

```
julia> plot(chn)
```

Note the characteristic “fuzzy caterpillar” look of the trace. This is what you expect to see (and what is absent in the OP, which tells me something is wrong there; I was unable to run your original model, however). A better check is to use `ArviZ.plot_rank`

:

```
julia> using ArviZ
julia> plot_rank(chn)
```

Approximately uniform histograms are what you hope to see, and we do.