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.
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