# Latent Variable Implementation#

The gp.Latent class is a direct implementation of a GP. It is called “Latent” because the underlying function values are treated as latent variables. It has a prior method, and a conditional method. Given a mean and covariance function, the function $$f(x)$$ is modeled as,

$f(x) \sim \mathcal{GP}(m(x),\, k(x, x')) \,.$

## .prior#

With some data set of finite size, the prior method places a multivariate normal prior distribution on the vector of function values, $$\mathbf{f}$$,

$\mathbf{f} \sim \text{MvNormal}(\mathbf{m}_{x},\, \mathbf{K}_{xx}) \,,$

where the vector $$\mathbf{m}$$ and the matrix $$\mathbf{K}_{xx}$$ are the mean vector and covariance matrix evaluated over the inputs $$x$$. Some sample code is,

import numpy as np
import pymc3 as pm

# A one dimensional column vector of inputs.
X = np.linspace(0, 1, 10)[:,None]

with pm.Model() as latent_gp_model:
# Specify the covariance function.

# Specify the GP.  The default mean function is Zero.
gp = pm.gp.Latent(cov_func=cov_func)

# Place a GP prior over the function f.
f = gp.prior("f", X=X)


By default, PyMC3 reparameterizes the prior on f under the hood by rotating it with the Cholesky factor of its covariance matrix. This helps to reduce covariances in the posterior of the transformed random variable, v. The reparameterized model is,

\begin{split} \begin{aligned} \mathbf{v} \sim \text{N}(0, 1)& \\ \mathbf{L} = \text{Cholesky}(\mathbf{K}_{xx})& \\ \mathbf{f} = \mathbf{m}_{x} + \mathbf{Lv} \\ \end{aligned} \end{split}

For more information about this reparameterization, see the section on drawing values from a multivariate distribution. This reparameterization can be disabled by setting the optional flag in the prior method, reparameterize = False. The default is True.

## .conditional#

The conditional method implements the predictive distribution for function values that were not part of the original data set. This distribution is,

$\mathbf{f}_* \mid \mathbf{f} \sim \text{MvNormal} \left( \mathbf{m}_* + \mathbf{K}_{*x}\mathbf{K}_{xx}^{-1} \mathbf{f} ,\, \mathbf{K}_{**} - \mathbf{K}_{*x}\mathbf{K}_{xx}^{-1}\mathbf{K}_{x*} \right)$

Using the same gp object we defined above, we can construct a random variable with this distribution by,

# vector of new X points we want to predict the function at
X_star = np.linspace(0, 2, 100)[:, None]

with latent_gp_model:
f_star = gp.conditional("f_star", X_star)


## Example 1: Regression with Student-T distributed noise#

The following is an example showing how to specify a simple model with a GP prior using the gp.Latent class. So we can verify that the inference we perform is correct, the data set is made using a draw from a GP.

import warnings

import arviz as az
import matplotlib.pyplot as plt
import numpy as np
import pymc3 as pm
import theano.tensor as tt

%config InlineBackend.figure_format = 'retina'
RANDOM_SEED = 8927
np.random.seed(RANDOM_SEED)
az.style.use("arviz-darkgrid")

# mute future warnings from theano
warnings.simplefilter(action="ignore", category=FutureWarning)

n = 200  # The number of data points
X = np.linspace(0, 10, n)[:, None]  # The inputs to the GP must be arranged as a column vector

# Define the true covariance function and its parameters
ℓ_true = 1.0
η_true = 3.0
cov_func = η_true**2 * pm.gp.cov.Matern52(1, ℓ_true)

# A mean function that is zero everywhere
mean_func = pm.gp.mean.Zero()

# The latent function values are one sample from a multivariate normal
# Note that we have to call eval() because PyMC3 built on top of Theano
f_true = np.random.multivariate_normal(
mean_func(X).eval(), cov_func(X).eval() + 1e-8 * np.eye(n), 1
).flatten()

# The observed data is the latent function plus a small amount of T distributed noise
# The standard deviation of the noise is sigma, and the degrees of freedom is nu
σ_true = 2.0
ν_true = 3.0
y = f_true + σ_true * np.random.standard_t(ν_true, size=n)

## Plot the data and the unobserved latent function
fig = plt.figure(figsize=(12, 5))
ax = fig.gca()
ax.plot(X, f_true, "dodgerblue", lw=3, label="True generating function 'f'")
ax.plot(X, y, "ok", ms=3, label="Observed data")
ax.set_xlabel("X")
ax.set_ylabel("y")
plt.legend();


The data above shows the observations, marked with black dots, of the unknown function $$f(x)$$ that has been corrupted by noise. The true function is in blue.

### Coding the model in PyMC3#

Here’s the model in PyMC3. We use a $$\text{Gamma}(2, 1)$$ prior over the lengthscale parameter, and weakly informative $$\text{HalfCauchy}(5)$$ priors over the covariance function scale, and noise scale. A $$\text{Gamma}(2, 0.1)$$ prior is assigned to the degrees of freedom parameter of the noise. Finally, a GP prior is placed on the unknown function. For more information on choosing priors in Gaussian process models, check out some of recommendations by the Stan folks.

with pm.Model() as model:
ℓ = pm.Gamma("ℓ", alpha=2, beta=1)
η = pm.HalfCauchy("η", beta=1)

cov = η**2 * pm.gp.cov.Matern52(1, ℓ)
gp = pm.gp.Latent(cov_func=cov)

f = gp.prior("f", X=X)

σ = pm.HalfCauchy("σ", beta=5)
ν = pm.Gamma("ν", alpha=2, beta=0.1)
y_ = pm.StudentT("y", mu=f, lam=1.0 / σ, nu=ν, observed=y)

trace = pm.sample(1000, chains=2, cores=1, return_inferencedata=True)

Auto-assigning NUTS sampler...
Sequential sampling (2 chains in 1 job)
NUTS: [ν, σ, f_rotated_, η, ℓ]

100.00% [2000/2000 05:54<00:00 Sampling chain 0, 0 divergences]
100.00% [2000/2000 04:20<00:00 Sampling chain 1, 2 divergences]
Sampling 2 chains for 1_000 tune and 1_000 draw iterations (2_000 + 2_000 draws total) took 616 seconds.
There were 2 divergences after tuning. Increase target_accept or reparameterize.
The estimated number of effective samples is smaller than 200 for some parameters.

# check Rhat, values above 1 may indicate convergence issues
n_nonconverged = int(np.sum(az.rhat(trace)[["η", "ℓ", "f_rotated_"]].to_array() > 1.03).values)
print("%i variables MCMC chains appear not to have converged." % n_nonconverged)

0 variables MCMC chains appear not to have converged.


### Results#

Below is the joint posterior of the two covariance function hyperparameters. The red lines show the true values that were used to draw the function from the GP.

fig, ax = plt.subplots(1, 1, figsize=(6, 6))

az.plot_pair(
trace,
var_names=["η", "ℓ"],
kind=["hexbin"],
ax=ax,
gridsize=25,
divergences=True,
)

ax.axvline(x=η_true, color="r")
ax.axhline(y=ℓ_true, color="r");


Below is the joint posterior of the parameters of the Student-t noise distribution, again, red lines mark the values used to generate the data.

fig, ax = plt.subplots(1, 1, figsize=(6, 6))

az.plot_pair(
trace,
var_names=["ν", "σ"],
kind=["hexbin"],
ax=ax,
gridsize=25,
divergences=True,
)

ax.axvline(x=ν_true, color="r")
ax.axhline(y=σ_true, color="r");

# plot the results
fig = plt.figure(figsize=(12, 5))
ax = fig.gca()

# plot the samples from the gp posterior with samples and shading
from pymc3.gp.util import plot_gp_dist

plot_gp_dist(ax, trace.posterior["f"][0, :, :], X)

# plot the data and the true latent function
ax.plot(X, f_true, "dodgerblue", lw=3, label="True generating function 'f'")
ax.plot(X, y, "ok", ms=3, label="Observed data")

# axis labels and title
plt.xlabel("X")
plt.ylabel("True f(x)")
plt.title("Posterior distribution over $f(x)$ at the observed values")
plt.legend();


As you can see by the red shading, the posterior of the GP prior over the function does a great job of representing both the fit, and the uncertainty caused by the additive noise. The result also doesn’t over fit due to outliers from the Student-T noise model.

### Using .conditional#

Next, we extend the model by adding the conditional distribution so we can predict at new $$x$$ locations. Lets see how the extrapolation looks out to higher $$x$$. To do this, we extend our model with the conditional distribution of the GP. Then, we can sample from it using the trace and the sample_posterior_predictive function. This is similar to how Stan uses its generated quantities {...} blocks. We could have included gp.conditional in the model before we did the NUTS sampling, but it is more efficient to separate these steps.

# 200 new values from x=0 to x=15
n_new = 200
X_new = np.linspace(0, 15, n_new)[:, None]

# add the GP conditional to the model, given the new X values
with model:
f_pred = gp.conditional("f_pred", X_new)

# Sample from the GP conditional distribution
with model:
pred_samples = pm.sample_posterior_predictive(trace.posterior, vars=[f_pred])

100.00% [2000/2000 06:16<00:00]
fig = plt.figure(figsize=(12, 5))
ax = fig.gca()
plot_gp_dist(ax, pred_samples["f_pred"], X_new)

ax.plot(X, f_true, "dodgerblue", lw=3, label="True generating function 'f'")
ax.plot(X, y, "ok", ms=3, label="Observed data")

ax.set_xlabel("X")
ax.set_ylabel("True f(x)")
ax.set_title("Conditional distribution of f_*, given f")
plt.legend();


## Example 2: Classification#

First we use a GP to generate some data that follows a Bernoulli distribution, where $$p$$, the probability of a one instead of a zero is a function of $$x$$. I reset the seed and added more fake data points, because it can be difficult for the model to discern variations around 0.5 with few observations.

import sys

# reset the random seed for the new example
RANDOM_SEED = 8927
np.random.seed(RANDOM_SEED)

# number of data points
n = 400

# x locations
x = np.linspace(0, 1.5, n)

# true covariance
ℓ_true = 0.1
η_true = 1.0
cov_func = η_true**2 * pm.gp.cov.ExpQuad(1, ℓ_true)
K = cov_func(x[:, None]).eval()

# zero mean function
mean = np.zeros(n)

# sample from the gp prior
f_true = np.random.multivariate_normal(mean, K + 1e-6 * np.eye(n), 1).flatten()

def invlogit(x, eps=sys.float_info.epsilon):
return (1.0 + 2.0 * eps) / (1.0 + np.exp(-x)) + eps

y = pm.Bernoulli.dist(p=invlogit(f_true)).random()

fig = plt.figure(figsize=(12, 5))
ax = fig.gca()

ax.plot(x, invlogit(f_true), "dodgerblue", lw=3, label="True rate")
# add some noise to y to make the points in the plot more visible
ax.plot(x, y + np.random.randn(n) * 0.01, "ko", ms=3, label="Observed data")

ax.set_xlabel("X")
ax.set_ylabel("y")
plt.legend();

with pm.Model() as model:
# informative lengthscale prior
ℓ = pm.Gamma("ℓ", alpha=2, beta=1)
# informative, positive normal prior on the period
η = pm.HalfNormal("η", sigma=5)
cov = η**2 * pm.gp.cov.ExpQuad(1, ℓ)

gp = pm.gp.Latent(cov_func=cov)

# make gp prior
f = gp.prior("f", X=x[:, None])

# logit link and Bernoulli likelihood
p = pm.Deterministic("p", pm.math.invlogit(f))
y_ = pm.Bernoulli("y", p=p, observed=y)

trace = pm.sample(1000, chains=2, cores=1, return_inferencedata=True)

Auto-assigning NUTS sampler...
Sequential sampling (2 chains in 1 job)
NUTS: [f_rotated_, η, ℓ]

100.00% [2000/2000 16:44<00:00 Sampling chain 0, 1 divergences]
100.00% [2000/2000 16:16<00:00 Sampling chain 1, 1 divergences]
Sampling 2 chains for 1_000 tune and 1_000 draw iterations (2_000 + 2_000 draws total) took 1981 seconds.
There was 1 divergence after tuning. Increase target_accept or reparameterize.
There were 2 divergences after tuning. Increase target_accept or reparameterize.
The number of effective samples is smaller than 25% for some parameters.

# check Rhat
n_nonconverged = int(np.sum(az.rhat(trace)[["η", "ℓ", "f_rotated_"]].to_array() > 1.03).values)
print("%i variables MCMC chains appear not to have converged." % n_nonconverged)

0 variables MCMC chains appear not to have converged.

fig, ax = plt.subplots(1, 1, figsize=(6, 6))

az.plot_pair(
trace,
var_names=["η", "ℓ"],
kind=["hexbin"],
ax=ax,
gridsize=25,
divergences=True,
)

ax.axvline(x=η_true, color="r")
ax.axhline(y=ℓ_true, color="r");

n_pred = 200
X_new = np.linspace(0, 2.0, n_pred)[:, None]

with model:
f_pred2 = gp.conditional("f_pred2", X_new)

with model:
pred_samples = pm.sample_posterior_predictive(trace.posterior, vars=[f_pred2])

100.00% [2000/2000 09:14<00:00]
# plot the results
fig = plt.figure(figsize=(12, 5))
ax = fig.gca()

# plot the samples from the gp posterior with samples and shading
plot_gp_dist(ax, invlogit(pred_samples["f_pred2"]), X_new)

# plot the data (with some jitter) and the true latent function
plt.plot(x, invlogit(f_true), "dodgerblue", lw=3, label="True f")
plt.plot(
x,
y + np.random.randn(y.shape[0]) * 0.01,
"ok",
ms=3,
alpha=0.5,
label="Observed data",
)

# axis labels and title
plt.xlabel("X")
plt.ylabel("True f(x)")
plt.title("Posterior distribution over $f(x)$ at the observed values")
plt.legend(loc="upper right");

%load_ext watermark
%watermark -n -u -v -iv -w

arviz 0.8.3
numpy 1.19.0
pymc3 3.9.1
last updated: Wed Jun 24 2020

CPython 3.8.3
IPython 7.15.0
watermark 2.0.2