Merge pull request #232 from pabloitu/brier_evaluations

Implementation of the Brier Score and its consistency test.

Author: Serra314

csep/core/brier_evaluations.py

@@ -0,0 +1,178 @@
+import numpy
+import numpy as np
+from scipy.stats import poisson
+
+from csep.models import EvaluationResult
+from csep.core.exceptions import CSEPCatalogException
+
+
+def _brier_score_ndarray(forecast, observations):
+    """ Computes the brier (binary) score for spatial-magnitude cells
+    using the formula:
+
+    Q(Lambda, Sigma) = 1/N sum_{i=1}^N (Lambda_i - Ind(Sigma_i > 0 ))^2
+
+    where Lambda is the forecast array, Sigma is the observed catalog, N the
+    number of spatial-magnitude cells and Ind is the indicator function, which
+    is 1 if Sigma_i > 0 and 0 otherwise.
+
+    Args:
+        forecast: 2d array of forecasted rates
+        observations: 2d array of observed counts
+    Returns
+        brier: float, brier score
+    """
+
+    prob_success = 1 - poisson.cdf(0, forecast)
+    brier_cell = np.square(prob_success.ravel() - (observations.ravel() > 0))
+    brier = -2 * brier_cell.sum()
+
+    for n_dim in observations.shape:
+        brier /= n_dim
+    return brier
+
+
+def _simulate_catalog(sim_cells, sampling_weights, random_numbers=None):
+    """
+    Simulates a catalog by sampling from the sampling_weights array.
+    Identical to binomial_evaluations._simulate_catalog
+
+    Args:
+        sim_cells:
+        sampling_weights:
+        random_numbers:
+
+    Returns:
+
+    """
+    sim_fore = numpy.zeros(sampling_weights.shape)
+
+    if random_numbers is None:
+        # Reset simulation array to zero, but don't reallocate
+        sim_fore.fill(0)
+        num_active_cells = 0
+        while num_active_cells < sim_cells:
+            random_num = numpy.random.uniform(0,1)
+            loc = numpy.searchsorted(sampling_weights, random_num,
+                                     side='right')
+            if sim_fore[loc] == 0:
+                sim_fore[loc] = 1
+                num_active_cells += 1
+    else:
+        # Find insertion points using binary search inserting
+        # to satisfy a[i-1] <= v < a[i]
+        pnts = numpy.searchsorted(sampling_weights, random_numbers,
+                                  side='right')
+        # Create simulated catalog by adding to the original locations
+        numpy.add.at(sim_fore, pnts, 1)
+
+    assert sim_fore.sum() == sim_cells, "simulated the wrong number of events!"
+    return sim_fore
+
+
+def _brier_score_test(forecast_data, observed_data, num_simulations=1000,
+                      random_numbers=None, seed=None, verbose=True):
+    """  Computes the Brier consistency test conditional on the total observed
+    number of events
+
+    Args:
+        forecast_data: 2d array of forecasted rates for spatial_magnitude cells
+        observed_data: 2d array of a catalog resampled to spatial_magnitude
+            cells
+        num_simulations: number of synthetic catalog simulations
+        random_numbers: numpy array of random numbers to use for simulation
+        seed: seed for random number generator
+        verbose: print status updates
+
+
+
+    """
+    # Array-masking that avoids log singularities:
+    forecast_data = numpy.ma.masked_where(forecast_data <= 0.0, forecast_data)
+
+    # set seed for the likelihood test
+    if seed is not None:
+        numpy.random.seed(seed)
+
+    # used to determine where simulated earthquake should
+    # be placed, by definition of cumsum these are sorted
+    sampling_weights = (numpy.cumsum(forecast_data.ravel()) /
+                        numpy.sum(forecast_data))
+
+    # data structures to store results
+    simulated_brier = []
+    n_active_cells = len(numpy.unique(numpy.nonzero(observed_data.ravel())))
+    num_cells_to_simulate = int(n_active_cells)
+
+    # main simulation step in this loop
+    for idx in range(num_simulations):
+        if random_numbers is None:
+            sim_fore = _simulate_catalog(num_cells_to_simulate,
+                                         sampling_weights)
+        else:
+            sim_fore = _simulate_catalog(num_cells_to_simulate,
+                                         sampling_weights,
+                                         random_numbers=random_numbers[idx, :])
+
+        # compute Brier score
+        current_brier = _brier_score_ndarray(forecast_data.data, sim_fore)
+
+        # append to list of simulated Brier score
+        simulated_brier.append(current_brier)
+
+        # just be verbose
+        if verbose:
+            if (idx + 1) % 100 == 0:
+                print(f'... {idx + 1} catalogs simulated.')
+
+    obs_brier = _brier_score_ndarray(forecast_data.data, observed_data)
+    # quantile score
+    qs = numpy.sum(simulated_brier <= obs_brier) / num_simulations
+
+    # float, float, list
+    return qs, obs_brier, simulated_brier
+
+
+def brier_score_test(gridded_forecast,
+                     observed_catalog,
+                     num_simulations=1000,
+                     seed=None,
+                     random_numbers=None,
+                     verbose=False):
+    """
+    Performs the Brier conditional test on a Gridded Forecast using an
+    Observed Catalog. Normalizes the forecast so the forecasted rate are
+    consistent with the observations. This modification eliminates the strong
+    impact differences in the number distribution have on the forecasted rates.
+    """
+
+    # grid catalog onto spatial grid
+    try:
+        _ = observed_catalog.region.magnitudes
+    except CSEPCatalogException:
+        observed_catalog.region = gridded_forecast.region
+    gridded_catalog_data = observed_catalog.spatial_magnitude_counts()
+
+    # simply call likelihood test on catalog data and forecast
+    qs, obs_brier, simulated_brier = _brier_score_test(
+        gridded_forecast.data,
+        gridded_catalog_data,
+        num_simulations=num_simulations,
+        seed=seed,
+        random_numbers=random_numbers,
+        verbose=verbose
+    )
+
+    # populate result data structure
+    result = EvaluationResult()
+    result.test_distribution = simulated_brier
+    result.name = 'Brier score-Test'
+    result.observed_statistic = obs_brier
+    result.quantile = qs
+    result.sim_name = gridded_forecast.name
+    result.obs_name = observed_catalog.name
+    result.status = 'normal'
+    result.min_mw = numpy.min(gridded_forecast.magnitudes)
+
+    return result
+

tests/test_evaluations.py

@@ -1,10 +1,12 @@
 import os
 import numpy
+import numpy as np
+import scipy
 import unittest
 
 import csep.core.poisson_evaluations as poisson
 import csep.core.binomial_evaluations as binary
-
+import csep.core.brier_evaluations as brier
 def get_datadir():
     root_dir = os.path.dirname(os.path.abspath(__file__))
     data_dir = os.path.join(root_dir, 'artifacts', 'Comcat')
@@ -115,5 +117,62 @@ def test_binomial_likelihood(self):
         numpy.testing.assert_allclose(simulated_ll[0], -7.921741654647629)
 
 
+class TestPoissonBrier(unittest.TestCase):
+
+    def __init__(self, *args, **kwargs):
+        super().__init__(*args, **kwargs)
+        self.seed = 1
+        self.forecast_data = numpy.array([[0.3, 0.2, 0.1], [0.2, 0.1, 0.1]])
+        self.observed_data = numpy.array([[0, 1, 2], [1, 1, 0]])
+
+    def test_brier_score_calculation(self):
+
+        # 1 bin
+        rate = 1
+        prob = 1 - scipy.stats.poisson.pmf(0, rate)
+        brier_score_hit = -2 * (prob - 1)**2
+        brier_score = brier._brier_score_ndarray(numpy.array([[rate]]),
+                                                 numpy.array([[1]]))
+        numpy.testing.assert_allclose(brier_score_hit, brier_score)
+
+        brier_score_nohit = -2 * prob**2
+        brier_score = brier._brier_score_ndarray(numpy.array([[rate]]),
+                                                 numpy.array([[0]]))
+        numpy.testing.assert_allclose(brier_score_nohit, brier_score)
+        # 2 bins
+        rate = np.array([1, 0.5])
+        hits = np.array([0, 1])
+        prob = 1 - scipy.stats.poisson.pmf(0, rate)
+        brier_score_2bins = (-2 * (prob - hits)**2).sum() / len(rate)
+        brier_score = brier._brier_score_ndarray(np.array(rate),
+                                                 np.array([hits]))
+        numpy.testing.assert_allclose(brier_score_2bins, brier_score)
+
+        hits = np.array([0, 0])
+        brier_score_2bins = (-2 * (prob - hits)**2).sum() / len(rate)
+        brier_score = brier._brier_score_ndarray(np.array(rate),
+                                                 np.array([hits]))
+        numpy.testing.assert_allclose(brier_score_2bins, brier_score)
+
+    def test_brier_test(self):
+
+        expected_sim = numpy.array([1, 1, 1, 1, 0, 0])
+        qs, obs_brier, simulated_brier = brier._brier_score_test(
+            self.forecast_data,
+            self.observed_data,
+            num_simulations=1,
+            seed=self.seed,
+            verbose=True)
+
+        probs = 1 - scipy.stats.poisson.pmf(0, self.forecast_data.ravel())
+        sim_brier_analytic = ((-2 * (probs - expected_sim)**2).sum() /
+                              len(probs))
+        numpy.testing.assert_allclose(simulated_brier[0], sim_brier_analytic)
+        obs_brier_analytic = (
+            (-2 * (probs - self.observed_data.ravel().astype(bool))**2).sum() /
+            len(probs))
+        numpy.testing.assert_allclose(obs_brier, obs_brier_analytic)
+
+
 if __name__ == '__main__':
     unittest.main()