Chi-square statistic with variance computed from model amplitudes.
The chi2modvar statistic was named CHI MVAR in CIAO 3.4.
The chi2xspecvar statistic is equivalent to chi2datavar ("ahelp chi2datavar"), except that the variance is estimated using the background and source model amplitudes rather than the observed counts data:
sigma(i)^2 = S(i) + [A(S)/A(B)]^2 B(i,off) ,
where B(i,off) is the background model amplitude in bin i of the off-source region. The help file on the chi-square statistic has more information, including definitions of the quantities in the equation: "ahelp chisquare" .
Note on Background Subtraction
The background should not be subtracted from the data when this statistic is used. chi2modvar underestimates the variance when fitting background-subtracted data.
sherpa> set_stat("chi2modvar") sherpa> show_stat() Statistic: Chi2ModVar
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.