Gaussian Process Taste-test

The scikit-learn package has a nice Gaussian Process example - but what is it doing? In this notebook, we review the mathematics of Gaussian Processes, and then 1) run the scikit-learn example, 2) do the same thing by-hand with numpy/scipy, and finally 3) use the GPy package to compare a few different kernels, on the same test dataset.

Super-brief Introduction to GPs

Let us look at the basics of Gaussian Processes in one dimension. See Rasmussen and Williams for a great, pedagogically smooth introduction to Gaussian Processes that will teach you everything you will need to get started.

We denote $\vec{x}=(x_0, \dots, x_N)$ a vector of 1D input values. A 1D Gaussian process $f$ is such that $f \sim \mathcal{GP} \ \Longleftrightarrow \ p(f(\vec{x}), f(\vec{x}'))\ \mathrm{is\ Gaussian} \ \forall \vec{x}, \vec{x}'$.

It is fully characterized by a mean function and a kernel, $$\begin{eqnarray*}m(\vec{x}) &=& \mathbb{E}[ f(\vec{x}) ]\\ k(\vec{x}, \vec{x}') &=& \mathbb{E}[ (f(\vec{x})-m(\vec{x}))(f(\vec{x}')-m(\vec{x}')) ]\end{eqnarray*}$$

Let us consider a noisy dataset $(\vec{x},\vec{y})$ with Gaussian homoskedastic errors $\epsilon$ that are Gaussian distributed with standard deviation $\sigma$. Fitting a Gaussian Process to this data is equivalent to considering a set of basis functions $\{\phi_i(x)\}$ and finding the optimal weights $\{\omega_i\}$, which we assume to be Gaussian distributed with some covariance $\Sigma$. It can also be thought of as fitting for an unknown correlated noise term in the data. $$\begin{eqnarray*} \vec{y} &=& f(\vec{x}) + \vec{\epsilon}\\ \vec{\epsilon} &\sim & \mathcal{N}(0,\sigma^2 I)\\ f(\vec{x}) &=& \sum_i \omega_i \phi_i(\vec{x}) = \vec{\omega}^T \vec{\phi}(\vec{x}) \\ \vec{\omega} &\sim & \mathcal{N}(0,\Sigma)\\ \end{eqnarray*}$$

In this case, the mean function is assumed to be zero, $m(\vec{x}) = 0$. (This is not actually very constraining, as Rasmussen and Williams explain, and it is not equivalent to assuming that the mean of $f$ is zero.)

There are as many weights as there are data points, which makes the function $f$ very flexible. The weights are constrained by their Gaussian distribution, though. Importantly, the kernel is fully characterized by the choice of basis functions, via

$$\quad k(\vec{x},\vec{x}') = \vec{\phi}(\vec{x})^T \Sigma\ \vec{\phi}(\vec{x}')$$

Picking a set of basis functions is equivalent to picking a kernel, and vice versa. In the correlated noise model interpretation its the kernel function that makes more sense. Typically a kernel will have a handful of hyper-parameters $\vec{\theta}$, that govern the shape of the basis function and correlation structure of the covariance matrix of the predictions. These hyper-parameters can in be inferred from the data, via their log likelihood:

$$\log p(\vec{y} | \vec{x},\vec{\theta}) = \frac{1}{2} \vec{y}^T K^{-1} \vec{y} - \frac{1}{2} \log |K| - \frac{n}{2} \log 2\pi$$

(Here, the matrix $K$ has elements $K_{ij} = k(x_i,x_j) + \sigma^2 \delta_{ij}$. Note that evaluating the likelihood for $\theta$ involves computing the determinant of the matrix $K$.) Fitting the hyper-parameters is often done by maximizing this likelihood - but that only gets you the "best-fit" hyper-parameters. Posterior samples of the hyper-parameters can be drawn by MCMC in the usual way.

For any given set of hyper-parameters, we can use the Gaussian Process to predict new outputs $\vec{y}^*$ at inputs $\vec{x}^*$. Thanks to the magic of Gaussian distributions and linear algebra, one can show that the posterior distribution for the process evaluated at new inputs $\vec{x}^*$ given a fit to the existing values $(\vec{x},\vec{y})$ is also Gaussian:

$$p( f(\vec{x}^*) | \vec{y}, \vec{x}, \vec{x}^* ) \ = \ \mathcal{N}(\bar{f}, \bar{k})$$

The mean of this PDF for $f(\vec{x}^*)$ is

$$\bar{f} \ =\ k(\vec{x}^*,\vec{x})[k(\vec{x},\vec{x}) + \sigma^2 I]^{-1} \vec{y}$$

and its covariance is

$$\bar{k} = k(\vec{x}^*,\vec{x}^*) - k(\vec{x},\vec{x}^*) [k(\vec{x},\vec{x}) + \sigma^2 I]^{-1}k(\vec{x},\vec{x}^*)^T$$

Once the kernel is chosen, one can fit the data and make predictions for new data in a single linear algebra operation. Note that multiple matrix inversions and multiplications are involved, so Gaussian Processes can be computationally very expensive - and that the weights are being optimized during the arithmetic calculation of $\bar{f}$.

Inferring the hyper-parameters of the kernel make GPs even more expensive, thanks to the determinant calculation involved.

To generate large numbers of predictions, one just makes a long vector $\vec{x}^*$. We'll see this in the code below, when generating smooth functions to plot through sparse and noisy data. The mean prediction $\bar{f}$ is linear in the input data $y$, which is quite remarkable.

The above is all the math you need to run Gaussian Processes in simple situations. Here is a list of more advanced topics that you should think about when applying Gaussian Processes to real data:

• Generalizing to multiple input dimensions (keeping one output dimension) is trivial, but the case of multiple outputs is not (partly because it is less natural).
• Choosing a physically motivated kernel or a kernel that simplifies the computation, for example by yielding sparse matrices.
• Parametrizing the kernel and/or the mean function and inferring these hyperparameters from the data.
• Using a small fraction of the data to make predictions. This is referred to as Sparse Gaussian Processes. Finding an optimal "summary" subset of the data is key.
• Gaussian Processes natively work with Gaussian noise / likelihood functions. With non-Gaussian cases, some analytical results are no longer valid (e.g. the marginal likelihood) but approximations exist.
• What if the inputs $\vec{x}$ have uncertainties? There are various way to deal with this, but this is much more intensive than normal Gaussian Processes.


In [1]:

%matplotlib inline
import numpy as np
from matplotlib import pyplot as plt
plt.style.use('seaborn-whitegrid')



Make Some Data



In [2]:

def f(x):
"""The function to predict."""
return x * np.sin(x)

def make_data(N, rseed=1):
np.random.seed(rseed)

# Create some observations with noise
X = np.random.uniform(low=0.1, high=9.9, size=N)
X = np.atleast_2d(X).T

y = f(X).ravel()
dy = 0.5 + 1.0 * np.random.random(y.shape)
noise = np.random.normal(0, dy)
y += noise

return X, y, dy

X, y, dy = make_data(20)



Gaussian Process Regression with Scikit-Learn



In [3]:

# Get the master version of scikit-learn; new GP code isn't in release
# This needs to compile things, so it will take a while...
# Uncomment the following:

# !pip install git+git://github.com/scikit-learn/scikit-learn.git




In [4]:

from sklearn.gaussian_process import GaussianProcessRegressor
from sklearn.gaussian_process.kernels import RBF as SquaredExponential
from sklearn.gaussian_process.kernels import ConstantKernel as Amplitude

# Instanciate a Gaussian Process model
kernel = Amplitude(1.0, (1E-3, 1E3)) * SquaredExponential(10, (1e-2, 1e2))

# Instantiate a Gaussian Process model
gp = GaussianProcessRegressor(kernel=kernel,
alpha=(dy / y)**2,  # fractional errors in data
n_restarts_optimizer=10)

# Fit to data using Maximum Likelihood Estimation of the hyper-parameters
gp.fit(X, y)




Out[4]:

GaussianProcessRegressor(alpha=array([  2.81880e-01,   1.06665e-01,   8.21349e+01,   1.85751e+00,
1.38706e+00,   7.71353e-01,   1.74396e-01,   1.49819e-01,
3.77392e-02,   6.54060e-02,   2.14450e-02,   7.14823e-02,
6.34041e+01,   2.98751e-02,   3.18792e-01,   6.80960e-02,
9.46038e-02,   7.69771e-02,   2.29495e-01,   1.00917e-01]),
copy_X_train=True, kernel=1**2 * RBF(length_scale=10),
n_restarts_optimizer=10, normalize_y=False,
optimizer='fmin_l_bfgs_b', random_state=None)




In [5]:

gp.kernel_




Out[5]:

3.69**2 * RBF(length_scale=0.887)




In [6]:

# note: gp.kernel is the initial kernel
#       gp.kernel_ (with an underscore) is the fitted kernel
gp.kernel_.get_params()




Out[6]:

{'k1': 3.69**2,
'k1__constant_value': 13.646552048519276,
'k1__constant_value_bounds': (0.001, 1000.0),
'k2': RBF(length_scale=0.887),
'k2__length_scale': 0.88652850735084876,
'k2__length_scale_bounds': (0.01, 100.0)}




In [7]:

# Mesh the input space for evaluations of the real function, the prediction and
# its MSE
x_pred = np.atleast_2d(np.linspace(0, 10, 1000)).T

# Make the prediction on the meshed x-axis (ask for MSE as well)
y_pred, sigma = gp.predict(x_pred, return_std=True)




In [8]:

def plot_results(X, y, dy, x_pred, y_pred, sigma):
fig = plt.figure(figsize=(8, 6))
plt.plot(x_pred, f(x_pred), 'k:', label=u'$f(x) = x\,\sin(x)$')
plt.errorbar(X.ravel(), y, dy, fmt='k.', markersize=10, label=u'Observations',
ecolor='gray')
plt.plot(x_pred, y_pred, 'b-', label=u'Prediction')
plt.fill(np.concatenate([x_pred, x_pred[::-1]]),
np.concatenate([y_pred - 1.9600 * sigma,
(y_pred + 1.9600 * sigma)[::-1]]),
alpha=.3, fc='b', ec='None', label='95% confidence interval')
plt.xlabel('$x$')
plt.ylabel('$f(x)$')
plt.ylim(-10, 20)
plt.legend(loc='upper left');

plot_results(X, y, dy, x_pred, y_pred, sigma)






Gaussian Processes by-hand

Let us run the same example but solving the Gaussian Process equations by hand. Let's use the kernel constructed with scikit-learn (because its parameters are optimized) And let's compute the Gaussian process manually using Scipy linalg



In [9]:

import scipy.linalg
KXX = gp.kernel_(X)
A = KXX + np.diag((dy/y)**2.)
L = scipy.linalg.cholesky(A, lower=True)
KXXp = gp.kernel_(x_pred, X)
KXpXp = gp.kernel_(x_pred)
alpha = scipy.linalg.cho_solve((L, True), y)
y_pred = np.dot(KXXp, alpha) + np.mean(y, axis=0)
v = scipy.linalg.cho_solve((L, True), KXXp.T)
y_pred_fullcov = KXpXp - KXXp.dot(v)
sigma = np.sqrt(np.diag(y_pred_fullcov))




In [10]:

plot_results(X, y, dy, x_pred, y_pred, sigma)






Quick kernel comparison with GPy

Let's now use the GPy package and compare a couple of kernels applied to our example. We'll optimize the parameters in each case. We not only plot the mean and std dev of the process but also a few samples. As you can see, they look very different, and the choice of kernel is critical!



In [11]:

import GPy

kernels = [GPy.kern.RBF(input_dim=1),
GPy.kern.Brownian(input_dim=1),
GPy.kern.Matern32(input_dim=1),
GPy.kern.Matern52(input_dim=1),
GPy.kern.Cosine(input_dim=1)]
names = ['Gaussian', 'Brownian', 'Mattern32', 'Matern52', 'ExpQuad', 'Cosine']

fig, axs = plt.subplots(3, 2, figsize=(12, 12), sharex=True, sharey=True)
axs = axs.ravel()

for i, k in enumerate(kernels):
m = GPy.models.GPRegression(X, y[:,None], kernel=k)
m.optimize()
m.plot_f(ax=axs[i], plot_data=True, samples=4, legend=False, plot_limits=[0, 10])
# plotting four samples of the GP posterior too
axs[i].errorbar(X, y, yerr=dy, fmt="o", c='k')
axs[i].set_title(names[i])
axs[i].plot(x_pred, f(x_pred), 'k:', label=u'$f(x) = x\,\sin(x)$')
fig.tight_layout()




/Users/bl/anaconda/lib/python2.7/site-packages/GPy/core/gp.py:488: RuntimeWarning:covariance is not positive-semidefinite.




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