          Chi-square statistic with variance computed from the data.

The chi2datavar statistic was named CHI DVAR in CIAO 3.4.

If the number of counts in each bin is large (> 5), then the shape of the Poisson distribution from which the counts are sampled tends asymptotically towards that of a Gaussian distribution, with variance

sigma(i)^2 = N(i,S) + [A(S)/A(B)]^2 N(i,B) .

The background term appears only if a background region is specified and background subtraction is done. The help file on the chi-square statistic has more information, including definitions of the additional quantities shown in the equation:

"ahelp chisquare" .

sherpa> set_stat("chi2datavar")
sherpa> show_stat()
Statistic: Chi2DataVar

Set the fitting statistic and then confirm the new value.

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

December 2013