calc_stat

Context: utilities

Synopsis

Calculate the fit statistic by data set ID

Syntax

calc_stat( [ id, [otherids]] )

The function calc_stat returns the value of the fit statistic for the data set ID(s) specified; the data set ID(s) supplied to calc_stat must be ready for fitting, i.e. it has been assigned a model to be fit. The calc_stat function can be called with no arguments, in which case the fit statistic for the default data set ready for fitting will be returned. If multiple data set IDs are given, then the simultaneous fit statistic is calculated for those data sets. The fit statistic value returned by calc_stat represents the initial fit statistic, which matches the "istatval" value returned by get_fit_results.

id, otherids - the id(s) of the dataset(s) to use; if not given, uses the default dataset id (id=1 by default, see "ahelp get_default_id"); otherids=None

Below are the fit statistics available in Sherpa:

chi2constvar	chi^2 with constant variance computed from the counts data.
chi2modvar	chi^2 with model amplitude variance.
chi2gehrels	chi^2 with gehrels method (default).
chi2datavar	chi^2 with data variance.
chi2xspecvar	chi^2 with data variance (XSPEC-style, variance = 1.0 if data less than or equal to 0.0)
cstat	CStat - A maximum likelihood function (XSPEC implementation of Cash).
cash	Cash - A maximum likelihood function.

Example 1

sherpa> calc_stat()

When called with no arguments, calc_stat() returns the fit statistic for the default data set, if it has been assigned a model to be fit.

Example 2

sherpa> calc_stat(1)
sherpa> calc_stat("src")

Supplying a specific data set ID to calc_stat - such as "1" or a the string "src" - will return the initial fit statistic for only that data set.

Example 3

sherpa> calc_stat(1,2)
sherpa> calc_stat("src", "bkg")

When supplied with multiple data set IDs, calc_stat returns the intial fit statistic produced by the simultaneous fit on those data sets.

Bugs

See the bugs pages on the Sherpa website for an up-to-date listing of known bugs.