We analyse the properties of the partitioned densities of states in mesoscopic transport problems. In terms of an imaginary-potential model, a concise derivation of the local partial density of states (LPDOS) is given. Using a numerical method to calculate the functional derivative of the scattering matrix, a detailed study of different kinds of decomposed densities of states in double-barrier structures is carried out. We find that the dynamical local density of states dn_αα(x)/dE and the global partial density of states (GPDOS) dN_αα/dE in the nonclassical regime cannot be neglected in full quantum calculations.