  We present the analysis of photometric, spectroscopic, and morphological
properties for differently selected samples of passive galaxies up to z=1
extracted from the zCOSMOS-20k spectroscopic survey. This analysis intends to
explore the dependence of galaxy properties on the selection criterion adopted,
study the degree of contamination due to star-forming outliers, and provide a
comparison between different commonly used selection criteria. We extracted
from the zCOSMOS-20k catalog six different samples of passive galaxies, based
on morphology, optical colors, specific star-formation rate, a best fit to the
observed spectral energy distribution, and a criterion that combines
morphological, spectroscopic, and photometric information. The morphological
sample has the higher percentage of contamination in colors, specific star
formation rate and presence of emission lines, while the red & passive ETGs
sample is the purest, with properties mostly compatible with no star formation
activity; however, it is also the less economic criterion in terms of
information used. The best performing among the other criteria are the red SED
and the quiescent ones, providing a percentage of contamination only slightly
higher than the red & passive ETGs criterion (on average of a factor of ~2) but
with absolute values of the properties of contaminants still compatible with a
red, passively evolving population. We also provided two revised definitions of
early type galaxies based on restframe color-color and color-mass criteria,
that better reproduce the observed bimodalities. The analysis of the number
densities shows evidences of mass-assembly downsizing, with galaxies at
10.25<log(M/Msun)<10.75 increasing their number by a factor ~2-4 from z=0.6 to
z=0.2, by a factor ~2-3 from z=1 to z=0.2 at 10.75<log(M/Msun)<11, and by only
~10-50% from z=1 to z=0.2 at 11<log(M/Msun)<11.5.
