Source code for gammapy.stats.poisson

# Licensed under a 3-clause BSD style license - see LICENSE.rst
"""
Poisson significance computations for these two cases.

* known background level ``mu_bkg``
* background estimated from ``n_off`
"""
from __future__ import absolute_import, division, print_function, unicode_literals
import numpy as np

__all__ = [
    'background',
    'background_error',
    'excess',
    'excess_error',
    'significance',
    'significance_on_off',
    'excess_matching_significance',
    'excess_matching_significance_on_off',
]

__doctest_skip__ = ['*']


[docs]def background(n_off, alpha): r"""Estimate background in the on-region from an off-region observation. .. math:: \mu_{background} = \alpha \times n_{off} Parameters ---------- n_off : array_like Observed number of counts in the off region alpha : array_like On / off region exposure ratio for background events Returns ------- background : ndarray Background estimate for the on region Examples -------- >>> background(n_off=4, alpha=0.1) 0.4 >>> background(n_off=9, alpha=0.2) 1.8 """ n_off = np.asanyarray(n_off, dtype=np.float64) alpha = np.asanyarray(alpha, dtype=np.float64) return alpha * n_off
[docs]def background_error(n_off, alpha): r"""Estimate standard error on background in the on region from an off-region observation. .. math:: \Delta\mu_{bkg} = \alpha \times \sqrt{n_{off}} Parameters ---------- n_off : array_like Observed number of counts in the off region alpha : array_like On / off region exposure ratio for background events Returns ------- background : ndarray Background estimate for the on region Examples -------- >>> background_error(n_off=4, alpha=0.1) 0.2 >>> background_error(n_off=9, alpha=0.2) 0.6 """ n_off = np.asanyarray(n_off, dtype=np.float64) alpha = np.asanyarray(alpha, dtype=np.float64) return alpha * np.sqrt(n_off)
[docs]def excess(n_on, n_off, alpha): r"""Estimate excess in the on region for an on-off observation. .. math:: \mu_{excess} = n_{on} - \alpha \times n_{off} Parameters ---------- n_on : array_like Observed number of counts in the on region n_off : array_like Observed number of counts in the off region alpha : array_like On / off region exposure ratio for background events Returns ------- excess : ndarray Excess estimate for the on region Examples -------- >>> excess(n_on=10, n_off=20, alpha=0.1) 8.0 >>> excess(n_on=4, n_off=9, alpha=0.5) -0.5 """ n_on = np.asanyarray(n_on, dtype=np.float64) n_off = np.asanyarray(n_off, dtype=np.float64) alpha = np.asanyarray(alpha, dtype=np.float64) return n_on - alpha * n_off
[docs]def excess_error(n_on, n_off, alpha): r"""Estimate error on excess for an on-off measurement. .. math:: \Delta\mu_{excess} = \sqrt{n_{on} + \alpha ^ 2 \times n_{off}} TODO: Implement better error and limit estimates (Li & Ma, Rolke)! Parameters ---------- n_on : array_like Observed number of counts in the on region n_off : array_like Observed number of counts in the off region alpha : array_like On / off region exposure ratio for background events Returns ------- excess_error : ndarray Excess error estimate Examples -------- >>> excess_error(n_on=10, n_off=20, alpha=0.1) 3.1937438845342623... >>> excess_error(n_on=4, n_off=9, alpha=0.5) 2.5 """ n_on = np.asanyarray(n_on, dtype=np.float64) n_off = np.asanyarray(n_off, dtype=np.float64) alpha = np.asanyarray(alpha, dtype=np.float64) variance = n_on + (alpha ** 2) * n_off return np.sqrt(variance)
# TODO: rename this function to something more explicit. # It currently has the same name as the `gammapy/stats/significance.py` # and shadows in in `gammapy/stats/__init.py` # Maybe `significance_poisson`?
[docs]def significance(n_on, mu_bkg, method='lima', n_on_min=1): r"""Compute significance for an observed number of counts and known background. The simple significance estimate :math:`S_{simple}` is given by .. math :: S_{simple} = (n_{on} - \mu_{bkg}) / \sqrt{\mu_{bkg}} The Li & Ma significance estimate corresponds to the Li & Ma formula (17) in the limiting case of known background :math:`\mu_{bkg} = \alpha \times n_{off}` with :math:`\alpha \to 0`. The following formula for :math:`S_{lima}` was obtained with Mathematica: .. math :: S_{lima} = \left[ 2 n_{on} \log \left( \frac{n_{on}}{\mu_{bkg}} \right) - n_{on} + \mu_{bkg} \right] ^ {1/2} Parameters ---------- n_on : array_like Observed number of counts mu_bkg : array_like Known background level method : str Select method: 'lima' or 'simple' n_on_min : float Minimum ``n_on`` (return ``NaN`` for smaller values) Returns ------- significance : ndarray Significance according to the method chosen. References ---------- .. [1] Li and Ma, "Analysis methods for results in gamma-ray astronomy", `Link <http://adsabs.harvard.edu/abs/1983ApJ...272..317L>`_ See Also -------- excess, significance_on_off Examples -------- >>> significance(n_on=11, mu_bkg=9, method='lima') 0.64401498442763649 >>> significance(n_on=11, mu_bkg=9, method='simple') 0.66666666666666663 >>> significance(n_on=7, mu_bkg=9, method='lima') -0.69397262486881672 >>> significance(n_on=7, mu_bkg=9, method='simple') -0.66666666666666663 """ n_on = np.asanyarray(n_on, dtype=np.float64) mu_bkg = np.asanyarray(mu_bkg, dtype=np.float64) if method == 'simple': func = _significance_simple elif method == 'lima': func = _significance_lima elif method == 'direct': func = _significance_direct else: raise ValueError('Invalid method: {}'.format(method)) # For low `n_on` values, don't try to compute a significance and return `NaN`. n_on = np.atleast_1d(n_on) mu_bkg = np.atleast_1d(mu_bkg) mask = (n_on >= n_on_min) s = np.ones_like(n_on) * np.nan s[mask] = func(n_on[mask], mu_bkg[mask]) return s
def _significance_simple(n_on, mu_bkg): # TODO: check this formula against ??? excess = n_on - mu_bkg bkg_err = np.sqrt(mu_bkg) return excess / bkg_err def _significance_lima(n_on, mu_bkg): sign = np.sign(n_on - mu_bkg) val = np.sqrt(2) * np.sqrt(n_on * np.log(n_on / mu_bkg) - n_on + mu_bkg) return sign * val def _significance_direct(n_on, mu_bkg): """Compute significance directly via Poisson probability. Use this method for small ``n_on < 10``. In this case the Li & Ma formula isn't correct any more. TODO: add large unit test coverage (where is it numerically precise enough)? TODO: check coverage with MC simulation I'm getting a positive significance for zero observed counts and small mu_bkg. That doesn't make too much sense ... >>> stats.poisson._significance_direct(0, 2) -1.1015196284987503 >>> stats.poisson._significance_direct(0, 0.1) 1.309617799458493 """ from scipy.stats import norm, poisson # Compute tail probability to see n_on or more counts probability = poisson.sf(n_on, mu_bkg) # Convert probability to a significance significance = norm.isf(probability) return significance
[docs]def significance_on_off(n_on, n_off, alpha, method='lima', neglect_background_uncertainty=False): r"""Compute significance of an on-off observation. TODO: describe available methods. Parameters ---------- n_on : array_like Observed number of counts in the on region n_off : array_like Observed number of counts in the off region alpha : array_like On / off region exposure ratio for background events method : {'lima', 'simple', 'direct'} Select method Returns ------- significance : array Significance according to the method chosen. References ---------- .. [1] Li and Ma, "Analysis methods for results in gamma-ray astronomy", `Link <http://adsabs.harvard.edu/abs/1983ApJ...272..317L>`_ See Also -------- significance, excess_matching_significance_on_off Examples -------- >>> significance_on_off(n_on=10, n_off=20, alpha=0.1, method='lima') 3.6850322319420274 >>> significance_on_off(n_on=10, n_off=20, alpha=0.1, method='simple') 2.5048971643405982 >>> significance_on_off(n_on=10, n_off=20, alpha=0.1, method='direct') 3.5281644971409953 """ n_on = np.asanyarray(n_on, dtype=np.float64) n_off = np.asanyarray(n_off, dtype=np.float64) alpha = np.asanyarray(alpha, dtype=np.float64) if method == 'simple': if neglect_background_uncertainty: mu_bkg = background(n_off, alpha) return _significance_simple(n_on, mu_bkg) else: return _significance_simple_on_off(n_on, n_off, alpha) elif method == 'lima': if neglect_background_uncertainty: mu_bkg = background(n_off, alpha) return _significance_lima(n_on, mu_bkg) else: return _significance_lima_on_off(n_on, n_off, alpha) elif method == 'direct': if neglect_background_uncertainty: mu_bkg = background(n_off, alpha) return _significance_direct(n_on, mu_bkg) else: return _significance_direct_on_off(n_on, n_off, alpha) else: raise ValueError('Invalid method: {}'.format(method))
def _significance_simple_on_off(n_on, n_off, alpha): r"""Compute significance with a simple, somewhat biased formula. .. math:: S = \mu_{excess} / \Delta\mu_{excess} where \mu_{excess} = n_{on} - \alpha \times n_{off} \Delta\mu_{excess} = \sqrt{n_{on} + \alpha ^ 2 \times n_{off}} Notes ----- This function implements formula (5) of Li & Ma. Li & Ma show that it is somewhat biased, but it does have the advantage of being analytically invertible, i.e. there is an analytical formula for the inverse, which is often used in practice as part of sensitivity computation. """ excess_ = excess(n_on, n_off, alpha) excess_error_ = excess_error(n_on, n_off, alpha) return excess_ / excess_error_ def _significance_lima_on_off(n_on, n_off, alpha): r"""Compute significance with the Li & Ma formula (17).""" sign = np.sign(excess(n_on, n_off, alpha)) tt = (alpha + 1) / (n_on + n_off) ll = n_on * np.log(n_on * tt / alpha) mm = n_off * np.log(n_off * tt) val = np.sqrt(np.abs(2 * (ll + mm))) return sign * val def _significance_direct_on_off(n_on, n_off, alpha): """Compute significance directly via Poisson probability. Use this method for small n_on < 10. In this case the Li & Ma formula isn't correct any more. * TODO: add reference * TODO: add large unit test coverage (where is it numerically precise enough)? * TODO: check coverage with MC simulation * TODO: implement in Cython and vectorize n_on (accept numpy array n_on as input) """ from math import factorial as fac from scipy.stats import norm # Compute tail probability to see n_on or more counts probability = 1 for n in range(0, n_on): term_1 = alpha ** n / (1 + alpha) ** (n_off + n + 1) term_2 = fac(n_off + n) / (fac(n) * fac(n_off)) probability -= term_1 * term_2 # Convert probability to a significance significance = norm.isf(probability) return significance
[docs]def excess_matching_significance(mu_bkg, significance, method='lima'): r"""Compute excess matching a given significance. This function is the inverse of `significance`. Parameters ---------- mu_bkg : array_like Known background level significance : array_like Significance method : {'lima', 'simple'} Select method Returns ------- excess : ndarray Excess See Also -------- significance, excess_matching_significance_on_off Examples -------- >>> excess_matching_significance(mu_bkg=0.2, significance=5, method='lima') TODO >>> excess_matching_significance(mu_bkg=0.2, significance=5, method='simple') TODO """ mu_bkg = np.asanyarray(mu_bkg, dtype=np.float64) significance = np.asanyarray(significance, dtype=np.float64) if method == 'simple': return _excess_matching_significance_simple(mu_bkg, significance) elif method == 'lima': return _excess_matching_significance_lima(mu_bkg, significance) else: raise ValueError('Invalid method: {}'.format(method))
def _excess_matching_significance_simple(mu_bkg, significance): raise NotImplementedError def _excess_matching_significance_lima(mu_bkg, significance): raise NotImplementedError
[docs]def excess_matching_significance_on_off(n_off, alpha, significance, method='lima'): r"""Compute sensitivity of an on-off observation. This function is the inverse of `significance_on_off`. Parameters ---------- n_off : array_like Observed number of counts in the off region alpha : array_like On / off region exposure ratio for background events significance : array_like Desired significance level method : {'lima', 'simple'} Which method? Returns ------- excess : `numpy.ndarray` Excess See Also -------- significance_on_off, excess_matching_significance Examples -------- >>> excess_matching_significance_on_off(n_off=20,alpha=0.1,significance=5,method='lima') 12.038 >>> excess_matching_significance_on_off(n_off=20,alpha=0.1,significance=5,method='simple') 27.034 >>> excess_matching_significance_on_off(n_off=20,alpha=0.1,significance=0,method='lima') 2.307301461e-09 >>> excess_matching_significance_on_off(n_off=20,alpha=0.1,significance=0,method='simple') 0.0 >>> excess_matching_significance_on_off(n_off=20,alpha=0.1,significance=-10,method='lima') nan >>> excess_matching_significance_on_off(n_off=20,alpha=0.1,significance=-10,method='simple') nan """ n_off = np.asanyarray(n_off, dtype=np.float64) alpha = np.asanyarray(alpha, dtype=np.float64) significance = np.asanyarray(significance, dtype=np.float64) if method == 'simple': return _excess_matching_significance_on_off_simple(n_off, alpha, significance) elif method == 'lima': return _excess_matching_significance_on_off_lima(n_off, alpha, significance) else: raise ValueError('Invalid method: {}'.format(method))
def _excess_matching_significance_on_off_simple(n_off, alpha, significance): # TODO: can these equations be simplified? significance2 = significance ** 2 determinant = significance2 + 4 * n_off * alpha * (1 + alpha) temp = significance2 + 2 * n_off * alpha n_on = 0.5 * (temp + significance * np.sqrt(np.abs(determinant))) return n_on - background(n_off, alpha) def _excess_matching_significance_on_off_lima(n_off, alpha, significance): from scipy.optimize import fsolve # Significance not well-defined for n_on < 0 # Return Nan if given significance can't be reached s0 = _significance_lima_on_off(n_on=1e-5, n_off=n_off, alpha=alpha) if s0 >= significance: return np.nan def target_significance(n_on): if n_on >= 0: return _significance_lima_on_off(n_on, n_off, alpha) - significance else: # This high value is to tell the optimiser to stay n_on >= 0 return 1e10 excess_guess = _excess_matching_significance_on_off_simple(n_off, alpha, significance) n_on_guess = excess_guess + background(n_off, alpha) # solver options to control robustness / accuracy / speed opts = dict(factor=0.1) n_on = fsolve(target_significance, n_on_guess, **opts) return n_on - background(n_off, alpha) _excess_matching_significance_on_off_lima = np.vectorize(_excess_matching_significance_on_off_lima)