# Example: Generalized Linear Mixed Models¶

The UCBadmit data is sourced from the study [1] of gender biased in graduate admissions at UC Berkeley in Fall 1973:

UCBadmit dataset
dept male applications admit
0 1 825 512
0 0 108 89
1 1 560 353
1 0 25 17
2 1 325 120
2 0 593 202
3 1 417 138
3 0 375 131
4 1 191 53
4 0 393 94
5 1 373 22
5 0 341 24

This example replicates the multilevel model m_glmm5 at [3], which is used to evaluate whether the data contain evidence of gender biased in admissions accross departments. This is a form of Generalized Linear Mixed Models for binomial regression problem, which models

• varying intercepts accross departments,
• varying slopes (or the effects of being male) accross departments,
• correlation between intercepts and slopes,

and uses non-centered parameterization (or whitening).

A more comprehensive explanation for binomial regression and non-centered parameterization can be found in Chapter 10 (Counting and Classification) and Chapter 13 (Adventures in Covariance) of [2].

References:

1. Bickel, P. J., Hammel, E. A., and O’Connell, J. W. (1975), “Sex Bias in Graduate Admissions: Data from Berkeley”, Science, 187(4175), 398-404.
2. McElreath, R. (2018), “Statistical Rethinking: A Bayesian Course with Examples in R and Stan”, Chapman and Hall/CRC.
3. https://github.com/rmcelreath/rethinking/tree/Experimental#multilevel-model-formulas
import argparse
import os

import matplotlib.pyplot as plt
import numpy as np

from jax import random
import jax.numpy as jnp
from jax.scipy.special import expit

import numpyro
import numpyro.distributions as dist
from numpyro.examples.datasets import UCBADMIT, load_dataset
from numpyro.infer import MCMC, NUTS, Predictive

def glmm(dept, male, applications, admit=None):
v_mu = numpyro.sample('v_mu', dist.Normal(0, jnp.array([4., 1.])))

sigma = numpyro.sample('sigma', dist.HalfNormal(jnp.ones(2)))
L_Rho = numpyro.sample('L_Rho', dist.LKJCholesky(2, concentration=2))
scale_tril = sigma[..., jnp.newaxis] * L_Rho
# non-centered parameterization
num_dept = len(np.unique(dept))
z = numpyro.sample('z', dist.Normal(jnp.zeros((num_dept, 2)), 1))
v = jnp.dot(scale_tril, z.T).T

logits = v_mu[0] + v[dept, 0] + (v_mu[1] + v[dept, 1]) * male
if admit is None:
# we use a Delta site to record probs for predictive distribution
probs = expit(logits)
numpyro.sample('probs', dist.Delta(probs), obs=probs)
numpyro.sample('admit', dist.Binomial(applications, logits=logits), obs=admit)

def run_inference(dept, male, applications, admit, rng_key, args):
kernel = NUTS(glmm)
mcmc = MCMC(kernel, args.num_warmup, args.num_samples, args.num_chains,
progress_bar=False if "NUMPYRO_SPHINXBUILD" in os.environ else True)
mcmc.run(rng_key, dept, male, applications, admit)
return mcmc.get_samples()

def print_results(header, preds, dept, male, probs):
columns = ['Dept', 'Male', 'ActualProb', 'Pred(p25)', 'Pred(p50)', 'Pred(p75)']
header_format = '{:>10} {:>10} {:>10} {:>10} {:>10} {:>10}'
row_format = '{:>10.0f} {:>10.0f} {:>10.2f} {:>10.2f} {:>10.2f} {:>10.2f}'
quantiles = jnp.quantile(preds, jnp.array([0.25, 0.5, 0.75]), axis=0)
print('\n', header, '\n')
print(header_format.format(*columns))
for i in range(len(dept)):
print(row_format.format(dept[i], male[i], probs[i], *quantiles[:, i]), '\n')

def main(args):
_, fetch_train = load_dataset(UCBADMIT, split='train', shuffle=False)
dept, male, applications, admit = fetch_train()
rng_key, rng_key_predict = random.split(random.PRNGKey(1))
zs = run_inference(dept, male, applications, admit, rng_key, args)
pred_probs = Predictive(glmm, zs)(rng_key_predict, dept, male, applications)['probs']
header = '=' * 30 + 'glmm - TRAIN' + '=' * 30
print_results(header, pred_probs, dept, male, admit / applications)

# make plots
fig, ax = plt.subplots(figsize=(8, 6), constrained_layout=True)

ax.plot(range(1, 13), admit / applications, "o", ms=7, label="actual rate")
ax.errorbar(range(1, 13), jnp.mean(pred_probs, 0), jnp.std(pred_probs, 0),
fmt="o", c="k", mfc="none", ms=7, elinewidth=1, label=r"mean $\pm$ std")
ax.plot(range(1, 13), jnp.percentile(pred_probs, 5, 0), "k+")
ax.plot(range(1, 13), jnp.percentile(pred_probs, 95, 0), "k+")
ax.set(xlabel="cases", ylabel="admit rate", title="Posterior Predictive Check with 90% CI")
ax.legend()

plt.savefig("ucbadmit_plot.pdf")

if __name__ == '__main__':
assert numpyro.__version__.startswith('0.6.0')
parser = argparse.ArgumentParser(description='UCBadmit gender discrimination using HMC')
parser.add_argument('-n', '--num-samples', nargs='?', default=2000, type=int)
parser.add_argument('--num-warmup', nargs='?', default=500, type=int)
parser.add_argument('--num-chains', nargs='?', default=1, type=int)
parser.add_argument('--device', default='cpu', type=str, help='use "cpu" or "gpu".')
args = parser.parse_args()

numpyro.set_platform(args.device)
numpyro.set_host_device_count(args.num_chains)

main(args)


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