The fit and goodness-of-fit test statistics are set using the
statistic command. Other fit-minimization algorithms are available,
and can be selected using the method command. The various fit methods
require first and in some cases second derivatives of the statistic
with respect to the parameters. By default XSPEC calculates these
analytically, using an approximation for the second derivatives. This
may be changed by setting the USE_CHAIN_RULE flag in
the user's startup Xspec.init file to `false`. The weighting algorithm used to
calculate can be altered by the weight command.

Other setting commands modify:

- cosmological parameters used to calculate luminosity (cosmo)
- solar abundances for 18 elements (abund)
- photoionization cross-sections (xsect)