statistic: change the objective function (statistic) for the fit

Change the fit statistic in use.

Syntax:           statistic[chi | cstat | lstat]

The options are chi-squared (chi) or C statistic (cstat) or lstat. Note that the C statistic has a couple of limitations: a) 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; b) the C statistic can be used to estimate parameter values and confidence regions but does not provide a goodness-of-fit. See Arnaud (2003, ApJ submitted) for further details.