The lowest electronic transitions in H₂, CH⁺ and Be have been analysed in terms of the individual contributions to excitation energies and transition moments in a second-order polarisation propagator calculation and in a second-order diagrammatic calculation. These are based on both the full particle-hole propagator matrix and on the single-transition approximation. Collective effects, defined as sums of infinite series of irreducible self-energy diagrams, do not affect the excitation energies but change the intensities by up to 40 per cent. The cancellation effects among the second-order contributions to the excitation energy are substantial, indicating the importance of using methods using methods which are consistent through a certain order in the electronic repulsion.