Constrained Hurdle Gamma Error

[zwelitunyiswa]

@tomicapretto I attempted to used the constrained function on a hurdle gamma model and got a "NotImplementedError: Truncation not implemented for SymbolicRandomVariable MarginalMixtureRV{inline=True}" error.

[tomicapretto]

Hi there! Could you share a basic example? To me, it looks like it's a PyMC thing. With an example I could reproduce the PyMC model and see if the problem is there.

[zwelitunyiswa]

@tomicapretto

No problem. Try this:

draws=20

dist_pre = pm.Gamma.dist(mu=9, sigma=10)
draws_pre = pm.draw(dist_pre, draws=40)

dist_post = pm.HurdleGamma.dist(psi=.7, mu=5, sigma=3)
draws_post = pm.draw(dist_post, draws=40)

groups = ['group1'] * 20 + ['group2'] * 20

data = pl.DataFrame({'group':groups, 'pre':draws_pre, 'post':draws_post}).with_columns(pl.all().exclude('group').round(2))

formula = bmb.Formula('constrained(post, 0, 30) ~ group + pre')

model = bmb.Model(formula=formula, data=data.to_pandas(), family='hurdle_gamma', link='log')

idata = model.fit()

idata

[GStechschulte]

Hey @zwelitunyiswa, I haven't ignored this. Things are a bit busy at work, but I will try to get to it soon. Thanks!

[tomicapretto]

The problem is that truncation is not implemented for mixtures such as the HurdleLogNormal distribution.

import numpy as np
import pymc as pm

dist_pre = pm.Gamma.dist(mu=9, sigma=10)
draws_pre = pm.draw(dist_pre, draws=40)

dist_post = pm.HurdleGamma.dist(psi=.7, mu=5, sigma=3)
draws_post = pm.draw(dist_post, draws=40)

groups = np.asarray(['group1'] * 20 + ['group2'] * 20)
group_names, group_idxs = np.unique(groups, return_inverse=True)
coords = {"group": group_names}

# This works
with pm.Model(coords=coords) as model:
    baseline = pm.Normal("baseline", dims="group")
    slope = pm.Normal("slope")

    mu = pm.math.exp(baseline[group_idxs] + slope * draws_pre)
    sigma = pm.HalfNormal("sigma")
    psi = pm.Beta("psi", alpha=2, beta=2)

    pm.HurdleLogNormal("post", psi=psi, mu=mu, sigma=sigma, observed=draws_post)

    idata = pm.sample(tune=100, draws=100, random_seed=1234)

# This does not work
with pm.Model(coords=coords) as model_censored:
    baseline = pm.Normal("baseline", dims="group")
    slope = pm.Normal("slope")

    mu = pm.math.exp(baseline[group_idxs] + slope * draws_pre)
    sigma = pm.HalfNormal("sigma")
    psi = pm.Beta("psi", alpha=2, beta=2)

    pm.Truncated(
        "post",
        pm.HurdleLogNormal.dist(psi=psi, mu=mu, sigma=sigma),
        lower=0,
        upper=30,
        observed=draws_post
    )

    idata = pm.sample(tune=100, draws=100, random_seed=1234)

On top of that, why do you use lower=0? The support of the distribution you're using already constrains values to be in the [0, infty) interval.