Chi-square statistic with variance set to 1.
The leastsq statistic is equivalent to chi2constvar ("ahelp chi2constvar"), except the variance is always set to 1.
sherpa> set_stat("leastsq") sherpa> show_stat()
Set the fitting statistic and then confirm the new value.
See the bugs pages on the Sherpa website for an up-to-date listing of known bugs.