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"
.
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.
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