Change the weighting function used in the calculation of chi-squared.
XSPEC> weight [standard | gehrels | churazov | model ]
Standard weighting uses 
or the statistical error given in the
input spectrum. Gehrels weighting uses 
, a better
approximation when N is small (Gehrels, N. 1986, ApJ 303, 336). Churazov
weighting uses the suggestion of Churazov etal (1996, ApJ 471, 673) to
estimate the weight for a given channel by averaging the counts in
surrounding channels. Model weighting uses the value of the model, not
the data, to estimate the weight.