# CashCountsStatistic¶

class gammapy.stats.CashCountsStatistic(n_on, mu_bkg)[source]

Bases: gammapy.stats.counts_statistic.CountsStatistic

Class to compute statistics (significance, asymmetric errors , ul) for Poisson distributed variable with known background.

Parameters
n_onint

Measured counts

mu_bkgfloat

Known level of background

Attributes Summary

 error Approximate error from the covariance matrix. n_bkg Expected background counts n_sig Excess p_value Return p_value of measured excess. sqrt_ts Return statistical significance of measured excess. stat_max Stat value for best fit hypothesis, i.e. stat_null Stat value for null hypothesis, i.e. ts Return stat difference (TS) of measured excess versus no excess.

Methods Summary

 compute_errn([n_sigma]) Compute downward excess uncertainties. compute_errp([n_sigma]) Compute upward excess uncertainties. compute_upper_limit([n_sigma]) Compute upper limit on the signal. n_sig_matching_significance(significance) Compute excess matching a given significance.

Attributes Documentation

error

Approximate error from the covariance matrix.

n_bkg

Expected background counts

n_sig

Excess

p_value

Return p_value of measured excess. Here the value accounts only for the positive excess significance (i.e. one-sided).

sqrt_ts

Return statistical significance of measured excess. The sign of the excess is applied to distinguish positive and negative fluctuations.

stat_max

Stat value for best fit hypothesis, i.e. expected signal mu = n_on - mu_bkg

stat_null

Stat value for null hypothesis, i.e. 0 expected signal counts

ts

Return stat difference (TS) of measured excess versus no excess.

Methods Documentation

compute_errn(n_sigma=1.0)

Compute downward excess uncertainties.

Searches the signal value for which the test statistics is n_sigma**2 away from the maximum.

Parameters
n_sigmafloat

Confidence level of the uncertainty expressed in number of sigma. Default is 1.

compute_errp(n_sigma=1)

Compute upward excess uncertainties.

Searches the signal value for which the test statistics is n_sigma**2 away from the maximum.

Parameters
n_sigmafloat

Confidence level of the uncertainty expressed in number of sigma. Default is 1.

compute_upper_limit(n_sigma=3)

Compute upper limit on the signal.

Searches the signal value for which the test statistics is n_sigma**2 away from the maximum or from 0 if the measured excess is negative.

Parameters
n_sigmafloat

Confidence level of the upper limit expressed in number of sigma. Default is 3.

n_sig_matching_significance(significance)

Compute excess matching a given significance.

This function is the inverse of significance.

Parameters
significancefloat

Significance

Returns
n_signumpy.ndarray

Excess