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Gradient of a Statistical Model

Gradient of a Statistical Model#

With version 0.1.6, Spey includes additional functionalities to extract gradient and Hessian information directly from the statistical model. The gradient and Hessian are defined as follows

\[ {\rm Gradient} = -\frac{d\log\mathcal{L}(\theta)}{d\theta}\quad , \quad {\rm Hessian} = -\frac{d^2\log\mathcal{L}(\theta)}{d\theta_i d\theta_j}\quad , \quad \mu ,\theta_i \in \theta \ . \]

In order to access this information we will use spey.math module.

Hide code cell content 
import spey
from spey.math import value_and_grad, hessian
import numpy as np
np.random.seed(14)

spey.math.value_and_grad() returns a function that computes negative log-likelihood and its gradient for a given statistical model and spey.math.hessian() returns a function that computes Hessian of negative log-likelihood.

Let us examine this on "default_pdf.uncorrelated_background":

pdf_wrapper = spey.get_backend("default_pdf.uncorrelated_background")

data = [36, 33]
signal_yields = [12.0, 15.0]
background_yields = [50.0, 48.0]
background_unc = [12.0, 16.0]

stat_model = pdf_wrapper(
    signal_yields=signal_yields,
    background_yields=background_yields,
    data=data,
    absolute_uncertainties=background_unc,
)

Here we constructed a two-bin statistical model with observations \(36,\ 33\), signal yields \(12,\ 15\) and background yields \(50\pm12,\ 48\pm16\). We can construct the function that will return negative log probability and its gradient as follows

neg_logprob = value_and_grad(stat_model)

Notice that this function constructs a negative log-probability for the observed statistical model using the default data that we provided earlier. This can be changed using expected and data keywords. Now we can choose nuisance parameters and execute the function:

nui = np.random.uniform(0,1,(3,))
neg_logprob(nui)

(27.81902589793928, array([13.29067478,  6.17223275,  9.28814191]))

For this particular model, we have only two nuisance parameters, \(\theta_i\), and signal strength, \(\mu\), due to the structure of the statistical model. For Hessian, we can use the same formulation:

hess = hessian(stat_model)
hess(nui)

array([[ 2.74153126,  1.21034187,  1.63326868],
       [ 1.21034187,  2.21034187, -0.        ],
       [ 1.63326868, -0.        ,  2.74215326]])