Growth rate dispersion (GRD) is the broadening of the crystal size distribution in crystallisation resulting from the inherent variability of growth rates between otherwise indistinguishable crystals. It is particularly large for sugar and presents a seriious limit on the uniformity of size from pans. It is proposed that GRD can be considered as a bivariate distribution in terms of the inherent growth rate and crystal size. A form of the population balance incorporating GRD is proposed. One special case is common history seed where, for all the crystals considered, the size is proportional to the inherent growth rate. For batch sugar crystallisations without nucleation, common history seeds give rise to a simple solution of the population balance and a simple method of modelling and analysis that could be incorporated into batch pan models.