  We collect data from the radio to the gamma-ray range for three complete
samples of blazars: the Slew Survey and the 1Jy samples of BL Lacs and the 2Jy
sample of Flat Spectrum Radio-Loud Quasars (FSRQs). The fraction of objects
detected in gamma-rays (E > 100 MeV) is 17%, 26% and 40% in the three samples
respectively. Except for the Slew Survey sample, gamma-ray detected sources do
not differ either from other sources in each sample, nor from all the gamma-ray
detected sources, in terms of the distributions of redshift, radio and X-ray
luminosities and of the broad band spectral indices (radio to optical and radio
to X-ray). We compute average Spectral Energy Distributions (SEDs) from radio
to gamma rays for groups of blazars binned according to radio luminosity,
irrespective of the original classification as BL Lac or FSRQ. The resulting
SEDs show a remarkable continuity in that: i) the first peak occurs in
different frequency ranges for different luminosity classes, with most luminous
sources peaking at lower frequencies; ii) the peak frequency of the gamma-ray
component correlates with the peak frequency of the lower energy one; iii) the
luminosity ratio between the high and low frequency components increases with
bolometric luminosity. The continuity of properties among different classes of
blazars and the systematic trends of the SEDs as a function of luminosity favor
a unified view of the blazar phenomenon: a single parameter, related to
luminosity, seems to govern the physical properties and radiation mechanisms in
the relativistic jets present in BL Lac objects as well as in FSRQ. The general
implications of this unified scheme are discussed.
