  A new method of evaluating relative (arm with respect to the interarm disk)
star formation rates and relative star formation efficiencies, together with
spiral arm-amplitudes, as a function of the galactocentric radius, using
broad-band photometry is derived. The classical method for obtaining star
formation rates from H_alpha photometry is discussed, and a new method is
derived for diagnosing the possible presence of biased star formation due to
different initial mass functions in the arms and in the interarm disk. As an
example, these methods are applied to the spiral galaxies NGC 4321 and NGC
4254, obtaining their arm amplitudes, relative arm/interarm star formation
efficiencies, and relative arm/interarm initial mass functions for each arm, as
a function of the galactocentric radius. Both objects present evidence of
massive star formation triggering in the spiral arms consistent with the spiral
density-wave theory, and a different initial mass function in the arms from
that in the rest of the disk, in the sense of favoring a larger fraction of
massive stars in the arms. This biased star formation is present in the zones
of the arms where there is triggering of massive star formation, and is then
related to, and probably caused, by the density-wave system. However, due to
this biased star formation, the spiral arms of the galaxies studied do not
trigger the formation of a larger total mass of stars (of all spectral types)
with respect to the interarm disk.
