## How can I specify which measurement errors to use in fitting a PHA data set?

If the measurement, or statistical, errors associated
with a PHA data set are present in the spectrum file to
be loaded into Sherpa, they can be loaded in association
with the PHA data values by using the `load_pha` command with its `use_errors` argument
set to "True". Otherwise, the
`load_staterror` command may be
used to read in an array of statistical errors.

For example:

sherpa> load_pha("src", "3c273.pi", use_errors=True) read ARF file 3c273.arf read RMF file 3c273.rmf read background file 3c273_bg.pi sherpa> get_staterror("src") array([ 4.12310563, 3.87298335, ...., 3.87298335, 4.47213595]) OR sherpa> load_pha("3c273.pi") WARNING: systematic errors were not found in file '3c273.pi' statistical errors were found in file '3c273.pi' but not used; to use them, re-read with use_errors=True read ARF file 3c273.arf read RMF file 3c273.rmf sherpa> load_staterror("src", "3c273.pi[cols channel,my_stat_err]") sherpa> get_staterror() array([ 3.8942, 3.4892, ...., 3.4892, 4.1527])

If statistical errors are input to Sherpa in association
with a data set by using `load_pha` or
`load_staterror` as demonstrated above, then Sherpa will
use these errors in the calculation of the statistic
value during a fit to the data set; if not, Sherpa will use the
statistical errors it calculates internally, based on
the currently chosen fit statistic (default is Chi2Gehrels).

For example, if no statistical errors are read into
Sherpa for data set "src", Sherpa
will use the errors returned by
`get_staterror`:

sherpa> load_pha("3c273.pi") WARNING: systematic errors were not found in file '3c273.pi' statistical errors were found in file '3c273.pi' but not used; to use them, re-read with use_errors=True read ARF file 3c273.arf read RMF file 3c273.rmf sherpa> show_stat() Statistic: Chi2Gehrels Chi Squared with Gehrels variance sherpa> get_staterror() array([ 5.21307489, 4.96862697, ..., 4.96862697, 5.55521679])