Using broad band photometry, a new method is derived to evaluate 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. The classical method to obtain star formation rates from Hα photometry is discussed, and a new method is inferred to diagnose the possible presence of biassed 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 galaxy NGC 4321. Evidence is obtained of massive star formation triggered in the spiral arms in a way consistent with the spiral density wave theory, and star formation biassed towards a larger fraction of massive stars in the arms than in the interarm disk, related to and probably caused by the density wave system.