Chi-square statistic with constant variance computed from the counts data.
The chi2constvar statistic was named CHI PARENT or CHI CVAR in CIAO 3.4.
In some applications, analysts have seen fit to assume that the variance is constant for each bin. For this choice of statistic, the variance is assumed to be the mean number of counts, or
sigma(i)^2 = (1/N) * (sum)_(j=1)^N N(j,S) + [A(S)/A(B)]^2 N(j,B) ,
where N is the number of on-source (and off-source) bins included in the fit. 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 in the equation: "ahelp chisquare" .
sherpa> set_stat("chi2constvar") sherpa> show_stat() Statistic: Chi2ConstVar
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.