According to the literature, one of the formation mechanisms of crystal growth rate dispersion consists in each individual crystal growing at an adequate constant rate, with different rates however for different crystals. In the present paper, this case has been described on the basis of a population density of nuclei, making use of the classical exponential dependence amongst others. Differences between n and n have been interpreted. The dependencies describing the population density of crystals n have been presented and a simplified method for their calculation proposed.