Change the fit statistic in use.
XSPEC> statistic [chi | cstat]
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.