  This article presents a study of the sufficient accuracy of post-Newtonian
and numerical relativity waveforms for the most demanding usage case: parameter
estimation of strong sources in advanced gravitational wave detectors. For
black hole binaries, these detectors require accurate waveform models which can
be constructed by fusing an analytical post-Newtonian inspiral waveform with a
numerical relativity merger-ringdown waveform. We perform a comprehensive
analysis of errors that enter such "hybrid waveforms". We find that the
post-Newtonian waveform must be aligned with the numerical relativity waveform
to exquisite accuracy, about 1/100 of a gravitational wave cycle. Phase errors
in the inspiral phase of the numerical relativity simulation must be controlled
to less than about 0.1rad. (These numbers apply to moderately optimistic
estimates about the number of GW sources; exceptionally strong signals require
even smaller errors.) The dominant source of error arises from the inaccuracy
of the investigated post-Newtonian Taylor-approximants. Using our error
criterium, even at 3.5-th post-Newtonian order, hybridization has to be
performed significantly before the start of the longest currently available
numerical waveforms which cover 30 gravitational wave cycles. The current
investigation is limited to the equal-mass, zero-spin case and does not take
into account calibration errors of the gravitational wave detectors.
