  With the improvement in sensitivity of gravitational wave (GW) detectors and
the increasing diversity of GW sources, there is a strong need for accurate GW
waveform models for data analysis. While the current model accuracy assessments
require waveforms generated by numerical relativity (NR) simulations as the
"true waveforms", in this paper we propose an assessment approach that does not
require NR simulations, which enables us to assess model accuracy everywhere in
the parameter space. By measuring the difference between two waveform models,
we derive a necessary condition for a pair of waveform models to both be
accurate, for a particular set of parameters. We then apply this method to the
parameter estimation samples of the Gravitational-Wave Transient Catalogs
GWTC-3 and GWTC-2.1, and find that the waveform accuracy for high
signal-to-noise ratio events in some cases fails our assessment criterion.
Based on analysis of real events' posterior samples, we discuss the correlation
between our quantified accuracy assessments and systematic errors in parameter
estimation. We find waveform models that perform worse in our assessment are
more likely to give inconsistent estimations. We also investigate waveform
accuracy in different parameter regions, and find the accuracy degrades as the
spin effects go up, the mass ratio deviates from one, or the orbital plane is
near-aligned to the line of sight. Furthermore, we make predictions of waveform
accuracy requirements for future detectors and find the accuracy of current
waveform models should be improved by at least 3 orders of magnitude, which is
consistent with previous works.
