Star formation rate and accumulated 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. Using a model galaxy population, we investigate possible causes for this discrepancy and conclude that minor errors in the conversion from observables to physical quantities can lead to a 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 the hierarchical galaxy formation theory.