  A main goal of NASA's Kepler Mission is to establish the frequency of
potentially habitable Earth-size planets (eta Earth). Relatively few such
candidates identified by the mission can be confirmed to be rocky via dynamical
measurement of their mass. Here we report an effort to validate 18 of them
statistically using the BLENDER technique, by showing that the likelihood they
are true planets is far greater than that of a false positive. Our analysis
incorporates follow-up observations including high-resolution optical and
near-infrared spectroscopy, high-resolution imaging, and information from the
analysis of the flux centroids of the Kepler observations themselves. While
many of these candidates have been previously validated by others, the
confidence levels reported typically ignore the possibility that the planet may
transit a different star than the target along the same line of sight. If that
were the case, a planet that appears small enough to be rocky may actually be
considerably larger and therefore less interesting from the point of view of
habitability. We take this into consideration here, and are able to validate 15
of our candidates at a 99.73% (3 sigma) significance level or higher, and the
other three at slightly lower confidence. We characterize the GKM host stars
using available ground-based observations and provide updated parameters for
the planets, with sizes between 0.8 and 2.9 Earth radii. Seven of them
(KOI-0438.02, 0463.01, 2418.01, 2626.01, 3282.01, 4036.01, and 5856.01) have a
better than 50% chance of being smaller than 2 Earth radii and being in the
habitable zone of their host stars.
