set_stat

sherpa.astro.ui.set_stat(stat)

Set the statistical method.

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

Parameters:stat (str) – The name of the statistic (case is not important). The list_stats function returns the list of supported values.
Raises:sherpa.utils.err.ArgumentErr – If the stat argument is not recognized.

See also

calc_stat()
Calculate the statistic value for a dataset.
get_stat_name()
Return the current statistic method.
list_stats()
List the supported fit statistics.
load_user_stat()
Create a user-defined statistic.

Notes

The available statistics include:

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.
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 (XSPEC-style, variance = 1.0 if data less than or equal to 0.0).
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

[1]Cash, W. “Parameter estimation in astronomy through application of the likelihood ratio”, ApJ, vol 228, p. 939-947 (1979). http://adsabs.harvard.edu/abs/1979ApJ...228..939C
[2]Gehrels, N. “Confidence limits for small numbers of events in astrophysical data”, ApJ, vol 303, p. 336-346 (1986). http://adsabs.harvard.edu/abs/1986ApJ...303..336G
[3](1, 2) https://heasarc.gsfc.nasa.gov/xanadu/xspec/manual/XSappendixStatistics.html

Examples

>>> set_stat('cash')