  In the quest for precision cosmology, one must ensure that the cosmology is
accurate as well. We discuss figures of merit for determining from observations
whether the dark energy is a cosmological constant or dynamical, with special
attention to the best determined equation of state value, at the ``pivot'' or
decorrelation redshift. We show this is not necessarily the best lever on
testing consistency with the cosmological constant, and moreover is subject to
bias. The standard parametrization of w(a)=w_0+w_a(1-a) by contrast is quite
robust, as tested by extensions to higher order parametrizations and modified
gravity. Combination of complementary probes gives strong immunization against
inaccurate, but precise, cosmology.
