05_BetaPosteriorPredictions.py

"""
Posterior predictive check. Examine the veracity of the winning model by
simulating data sampled from the winning model and see if the simulated data
'look like' the actual data.
"""
import numpy as np
from scipy.stats import beta
import matplotlib.pyplot as plt
plt.style.use('seaborn-darkgrid')

# Specify known values of prior and actual data.
prior_a = 100
prior_b = 1
actual_data_Z = 8
actual_data_N = 12
# Compute posterior parameter values.
post_a = prior_a + actual_data_Z
post_b = prior_b + actual_data_N - actual_data_Z
# Number of flips in a simulated sample should match the actual sample size:
sim_sample_size = actual_data_N
# Designate an arbitrarily large number of simulated samples.
n_sim_samples = 1000
# Set aside a vector in which to store the simulation results.
sim_sample_Z_record = np.zeros(n_sim_samples)
# Now generate samples from the posterior.
for sample_idx in range(0, n_sim_samples):
    # Generate a theta value for the new sample from the posterior.
    sample_theta = beta.rvs(post_a, post_b)
    # Generate a sample, using sample_theta.
    sample_data = np.random.choice([0, 1], p=[1-sample_theta, sample_theta],
                                  size=sim_sample_size, replace=True)
    sim_sample_Z_record[sample_idx] = sum(sample_data)


## Make a histogram of the number of heads in the samples.
plt.hist(sim_sample_Z_record)
plt.show()