Change the fit statistic in use.
XSPEC> statistic (chi or ml)
The options are chi-squared (chi) or C-statistic (cstat). Note
that the C-statistic has a couple of limitations : a) it
cannot be used on background-subtracted data; b) if the calculated model
goes to zero in any channel, then the format statistic goes to infinity -
in practice this is trapped and a large value of the statistic should
result.