    Set the statistical method.

set_stat(stat)
stat - str or sherpa.stats.Stat instance

Changes the method used to evaluate the fit statistic, that is the numerical measure that determines how closely the model represents the data.

>>> set_stat('cash')

PARAMETERS

The parameter for this function is:

Parameter
Definition
stat
When a string, the name of the statistic (case is not important): see `list_stats()` for supported values. Otherwise an instance of the statistic to use.

Notes

The available statistics include:

Item
Definition
cash
A maximum likelihood function [1].
chi2
Chi-squared statistic using the supplied error values.
chi2constvar
Chi-squared with constant variance computed from the counts data.
chi2datavar
Chi-squared with data variance. If the data has 0 counts then the error for that bin is 0.
chi2gehrels
Chi-squared with gehrels method [2]. This is the default method.
chi2modvar
Chi-squared with model amplitude variance.
chi2xspecvar
Chi-squared with data variance to match XSPEC. Errors from zero-count channels (source or background) are ignored if the other channel (background or source) contains counts, or replaced by a minimum value (when both source or background are empty). It should not be used when a model is fit to the background rather than the background is subtracted from the data.
cstat
A maximum likelihood function (the XSPEC implementation of the Cash function) [3]. This does not include support for including the background.
wstat
A maximum likelihood function which includes the background data as part of the fit (i.e. for when it is not being explicitly modelled) (the XSPEC implementation of the Cash function) [3].
leastsq
The least-squares statisic (the error is not used in this statistic).

References

Cash, W. "Parameter estimation in astronomy through application of the likelihood ratio", ApJ, vol 228, p. 939-947 (1979).
Gehrels, N. "Confidence limits for small numbers of events in astrophysical data", ApJ, vol 303, p. 336-346 (1986).
https://heasarc.gsfc.nasa.gov/xanadu/xspec/manual/XSappendixStatistics.html

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

December 2023