set_stat
- sherpa.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 or sherpa.stats.Stat instance) – 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.- 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. 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
- 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')