load_user_stat

Synopsis

Create a user-defined statistic.

Syntax

load_user_stat(statname, calc_stat_func, calc_err_func=None, priors={})

Description

The choice of statistics - that is, the numeric value that is minimised during the fit - can be extended by providing a function to calculate a numeric value given the data. The statistic is given a name and then can be used just like any of the pre-defined statistics.

Example

Define a chi-square statistic with the label "qstat":

>>> def qstat(d, m, staterr=None, syserr=None, w=None):
...     if staterr is None:
...         staterr = 1
...     c = ((d-m) / staterr)
...     return ((c*c).sum(), c)
...
>>> load_user_stat("qstat", qstat)
>>> set_stat("qstat")

PARAMETERS

The parameters for this function are:

Parameter	Type information	Definition
statname	str	The name to use for the new statistic when calling `set_stat` .
calc_stat_func	func	The function that calculates the statistic.
calc_err_func	func, optional	How to calculate the statistical error on a data point.
priors	dict	A dictionary of hyper-parameters for the priors.

Notes

The calc_stat_func should have the following signature:

def func(data, model, staterror=None, syserrr=None, weight=None)

where data is the array of dependent values, model the array of the predicted values, staterror and syserror are arrays of statistical and systematic errors respectively (if valid), and weight an array of weights. The return value is the pair (stat_value, stat_per_bin), where stat_value is a scalar and stat_per_bin is an array the same length as data.

The calc_err_func should have the following signature:

def func(data)

and returns an array the same length as data.

Bugs

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