We investigate the dependence on data quality of quasar properties measured from the C iv emission line region at high redshifts .
Our measurements come from 32 epochs of Sloan Digital Sky Survey ( SDSS ) Reverberation Mapping Project spectroscopic observations of 482 z > 1.46 quasars .
We compare the differences between measurements made from the single-epoch and coadded spectra , focusing on the C iv \lambda 1549 emission line because of its importance for studies of high-redshift quasar demographics and physical properties , including black hole masses .
In addition to increasing statistical errors ( by factors of \sim 2 - 4 ) , we find increasing systematic offsets with decreasing S/N .
The systematic difference ( measurement uncertainty ) in our lowest S/N ( < 5 ) subsample between the single-epoch and coadded spectrum ( i ) C iv equivalent width is 17Å ( 31Å ) , ( ii ) centroid wavelength is < 1Å ( 2Å ) , and fractional velocity widths , \Delta V / V , characterized by ( iii ) the line dispersion , \sigma _ { l } , is 0.104 ( 0.12 ) , and ( iv ) the mean absolute deviation ( MAD ) is 0.072 ( 0.11 ) .
These remain smaller than the 1 \sigma measurement uncertainties for all subsamples considered .
The MAD is found to be the most robust line-width characterization .
Offsets in the C iv full-width at half maximum ( FWHM ) velocity width and the C iv profile characterized by FWHM/ \sigma _ { l } are only smaller than the statistical uncertainties when S/N > 10 , although offsets in lower S/N spectra exceed the statistical uncertainties by only a factor of \sim 1.5 .
Characterizing the C iv line profile by the kurtosis is the least robust property investigated , as the median systematic coadded–single-epoch measurement differences are larger than the statistical uncertainties for all S/N subsamples .