  We investigate the dependence on data quality of quasar properties measured
from the CIV 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 CIV emission line because of its importance for studies of high-redshift
quasar demographics and physical properties, including black hole masses. In
addition to statistical errors increasing (by factors of ~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) CIV equivalent width is 17A (31A), (ii)
centroid wavelength is <1A (2A), 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 CIV
full-width at half maximum (FWHM) velocity width and the CIV 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 ~1.5. Characterizing the CIV 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.
