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Last modified: December 2010

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AHELP for CIAO 4.3 Sherpa v1


Context: utilities


Calculate the Chi Squared statistic contribution per bin


calc_chisqr( [ id, [otherids]] )


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)

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.


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

See Also

list_model_ids, show_bkg_model, show_bkg_source
get_iter_method_name, get_iter_method_opt, list_iter_methods, set_iter_method, set_iter_method_opt
add_model, add_user_pars, clean, create_model_component, delete_bkg_model, delete_model, delete_model_component, get_model, get_model_autoassign_func, get_model_component, get_model_component_image, get_model_component_plot, get_model_plot, get_num_par, get_order_plot, get_par, get_pileup_model, get_source, get_source_component_image, get_source_component_plot, image_model, image_model_component, image_source, image_source_component, integrate, link, load_table_model, load_user_model, reset, save_model, save_source, set_bkg_model, set_full_model, set_model_autoassign_func, set_pileup_model, set_source, set_xsabund, set_xscosmo, set_xsxsect, set_xsxset
plot_fit, plot_model, plot_model_component, plot_source, plot_source_component
delete_psf, load_conv
save_delchi, save_resid
calc_energy_flux, calc_model_sum, calc_photon_flux, calc_source_sum, calc_stat, eqwidth, gamma, igam, igamc, incbet, lgam
contour_model, contour_ratio, contour_resid, get_ratio, get_resid, image_fit

Last modified: December 2010
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