  We study an extensive sample of 87 GRBs for which there are well sampled and
simultaneous optical and X-ray light-curves. We extract the cleanest possible
signal of the afterglow component, and compare the temporal behaviors of the
X-ray light-curve, observed by Swift XRT, and optical data, observed by UVOT
and ground-based telescopes for each individual burst. Overall we find 62\%
GRBs that are consistent with the standard afterglow model. When more advanced
modeling is invoked, up to 91\% of the bursts in our sample may be consistent
with the external shock model. A large fraction of these bursts are consistent
with occurring in a constant interstellar density medium (ISM) (61\%) while
only 39\% of them occur in a wind-like medium. Only 9 cases have afterglow
light-curves that exactly match the standard fireball model prediction, having
a single power law decay in both energy bands which are observed during their
entire duration. In particular, for the bursts with chromatic behavior
additional model assumptions must be made over limited segments of the
light-curves in order for these bursts to fully agree with the external shock
model. Interestingly, for 54\% of the X-ray and 40\% of the optical band
observations the end of the shallow decay ($t^{\sim-0.5}$) period coincides
with the jet break ($t^{\sim-p}$) time, causing an abrupt change in decay
slope. The fraction of the burst that consistent with the external shock model
is independent of the observational epochs in the rest frame of GRBs. Moreover,
no cases can be explained by the cooling frequency crossing the X-ray or
optical band.
