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In the late seventies the spectral index dependence of the properties of steep
spectrum 3C and 4C sources were investigated in more detail by Blumenthal and Miley (1979).
They calculated the fractions of these sources with apparent magnitude smaller
than 
, with redshifts smaller than 
, with 178 MHz radio
luminosities smaller than 
and angular size larger than 
.
In both samples they found correlations between these parameters and the
spectral index.
They defined:
-
as the fraction of sources having observed 178 MHz
flux densities greater than 
with visual magnitudes smaller than
.
-
as the fraction of sources having observed 178 MHz
flux densities greater than 
with redshifts smaller than 
.
-
as the fraction of sources having observed 178 MHz
flux densities greater than 
with emitted 178 MHz monochromatic
luminosities smaller than 
.
-
as the fraction of sources having observed 178 MHz
flux densities greater than 
with angular sizes larger than 
.
Both samples were divided in smaller groups which have similar spectral
indices.