  The acceleration of the expansion of the universe, ascribed to a dark energy,
is one of the most intriguing discoveries in science. In addition to precise,
systematics controlled data, clear, robust interpretation of the observations
is required to reveal the nature of dark energy. Even for the simplest
question: is the data consistent with the cosmological constant? there are
important subtleties in the reconstruction of the dark energy properties. We
discuss the roles of analysis both in terms of the Hubble expansion rate or
dark energy density \rho_{DE}(z) and in terms of the dark energy equation of
state w(z), arguing that each has its carefully defined place. Fitting the
density is best for learning about the density, but using it to probe the
equation of state can lead to instability and bias.
