  We present a statistical analysis of the ultraviolet emission lines of
cataclysmic variables (CVs) based on $\approx 430$ ultraviolet spectra of 20
sources extracted from the International Ultraviolet Explorer Uniform Low
Dispersion Archive. These spectra are used to measure the emission line fluxes
of N V, Si IV, C IV, and He II and to construct diagnostic flux ratio diagrams.
We investigate the flux ratio parameter space populated by individual CVs and
by various CV subclasses (e.g., AM Her stars, DQ Her stars, dwarf novae,
nova-like variables). For most systems, these ratios are clustered within a
range of $\sim 1$ decade for log Si IV/C IV $\approx -0.5$ and log He II/C IV
$\approx -1.0$ and $\sim 1.5$ decades for log N V/C IV $\approx -0.25$. These
ratios are compared to photoionization and collisional ionization models to
constrain the excitation mechanism and the physical conditions of the
line-emitting gas. We find that the collisional models do the poorest job of
reproducing the data. The photoionization models reproduce the Si IV/C IV line
ratios for some shapes of the ionizing spectrum, but the predicted N V/C IV
line ratios are simultaneously too low by typically $\sim 0.5$ decades. Worse,
for no parameters are any of the models able to reproduce the observed He II/C
IV line ratios; this ratio is far too small in the collisional and scattering
models and too large by typically $\sim 0.5$ decades in the photoionization
models.
