calc_chisqr

Synopsis

Calculate the Chi Squared statistic contribution per bin

Syntax

calc_chisqr( [ id, [otherids]] )

Description

When fitting data with Chi Squared statistics, the calc_chisqr function returns an array of Chi Squared statistic contributions for each bin of the specified data set ID(s). The data set ID(s) supplied to calc_chisqr must be ready for fitting, i.e. it has been assigned a model to be fit. The calc_chisqr function can be called with no arguments, in which case the Chi Squared statistic array for the first data set ready for fitting will be returned. If multiple data set IDs are given, then the simultaneous Chi Squared fit statistics are calculated for those data sets.

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 Chi Squared 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)

Examples

Example 1

sherpa> calc_chisqr()

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

Example 2

sherpa> calc_chisqr(1)
sherpa> calc_chisqr("src")

Supplying a specific data set ID to calc_chisqr - such as "1" or "src" - will return the Chi Squared statistic array for only that data set.

Example 3

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

When supplied with multiple data set IDs, calc_chisqr returns the Chi Squared statistic array 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.