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What to do when you have small numbers of counts

The tex2html_wrap_inline10235 statistic assumes that all the spectral channels are Gaussian distributed. If there are small numbers of counts in a channel this will not be true. An alternative statistic, the C-statistic, can be used in this case provided that the data has not been background subtracted. The C-statistic now in use does provide a goodness-of-fit and this can be checked using the goodness command which uses Monte Carlo methods. This C-statistic can also provide confidence intervals in exactly the same way as tex2html_wrap_inline10235 . So give the command stat cstat and then use the fit and error commands as usual. An alternative approach is to continue using the tex2html_wrap_inline10235 statistic but change the weighting to provide a better approximation to a Gaussian distribution in the small number limit. This can be done using the weight gehrels or weight Churazov commands.



Keith Arnaud (kaa@genji.gsfc.nasa.gov)
Wed May 28 10:59:33 EDT 1997