We have studied the performance of global \chi ^ { 2 } fitting of low-resolution X-ray spectra in retrieving intrinsic source parameters , with emphasis on the coronal metallicity .
The study has been conducted by fitting large numbers of simulated spectra with known characteristics , and studying the distribution of best-fit parameters .
We have studied the behavior of the LECS detector on board the SAX satellite and the SIS detector on board the ASCA satellite .
The fitted source spectra have either two discrete temperature components or a power-law temperature distribution , with metallicity variations modeled by a single global abundance parameter .
The model used for the fitting has always been a two-temperature one , with global varying abundance , to explore the influence of the a priori ignorance of the actual temperature stratification in the source being observed .

The simulations performed explore the influence of varying statistics in the observed spectrum ( spanning a realistic range of values ) as well as the effect of varying the intrinsic source metallicity , with values in the range 0.15–1.0 times the solar value .
We find that the source metallicity can be retrieved within few tens of percent from ASCA/SIS spectra of typical signal to noise ratio , and within few percent from SAX/LECS spectra at the same signal to noise ratio .
However relatively small uncertainties in the detector calibrations and in the plasma emission codes are likely to potentially cause large systematic off-sets in the value of the best-fit parameters .
Similar systematic off-sets may derive from assuming too simplistic a temperature distribution for the source plasma .

In addition we have re-analyzed the ASCA/SIS spectra of the active giants \beta Cet and Capella with the same set of assumptions used in the simulations , showing how the best-fit metallicity in these two real cases depends on the details of the fitting process , and in particular on the chosen energy range .