  Star formation rate and accummulated stellar mass are two fundamental
physical quantities that describe the evolutionary state of a forming galaxy.
Two recent attempts to determine the relationship between these quantities, by
interpreting a sample of star-forming galaxies at redshift of z~4, have led to
opposite conclusions. We use a model galaxy population to investigate possible
causes for this discrepancy and conclude that minor errors in the conversion
from observables to physical quantities can lead to major misrepresentation
when applied without awareness of sample selection. We also investigate, in a
general way, the physical origin of the correlation between star formation rate
and stellar mass within hierarchical galaxy formation theory.
