We perform an extensive analysis of the C iv \lambda 1549 line in three large spectroscopic surveys of quasars .
Differing approaches for fitting the C iv line can be found in the literature , and we compare the most common methods to highlight the relative systematics associated with each .
We choose the line fitting procedure that results in a symmetric profile for the C iv line and gives accurate fits to local emission features around the line , and use this approach to measure the width of the C iv line in spectra from the SDSS , 2QZ and 2SLAQ surveys .

The results are compared with a previous study of the Mg ii \lambda 2799 line in the same sample .
We find the C iv line tends to be broader than the Mg ii line in spectra that have both lines , and the average ratio between the lines is consistent with a simplistic model for a photoionised , virialised and stratified broad-line region .
There exists a statistically significant correlation between the widths of the C iv and Mg ii lines .
However , the correlation is weak , and the scatter around a best fit is only marginally less than the full dynamic range of line widths .

Motivated by previous work on the Mg ii line , we examine the dispersion in the distribution of C iv line widths .
We find that the dispersion in C iv line widths is essentially independent of both redshift and luminosity .
This result is in stark contrast to the Mg ii line , which shows a strong luminosity dependence .
Furthermore we demonstrate that the low level of dispersion in C iv line width ( \sim 0.08 dex ) is inconsistent with a pure-disk model for the emitting region and use our data to constrain simple models for the broad-line region .

Finally we consider our results in terms of their implications for the the virial technique for estimating black hole masses .
The inconsistency between Mg ii and C iv line widths in single spectra , combined with the differing behaviour of the Mg ii and C iv line width distributions as a whole , indicates that there must be an inconsistency between Mg ii and C iv virial mass estimators .
Furthermore , the level of intrinsic dispersion in Mg ii and C iv line widths contributes less dynamic range to virial mass estimates than the error associated with the estimates .
The indication is that the line width term in these UV virial mass estimators may be essentially irrelevant with respect to the typical uncertainty on a mass estimate .