The temporal and spatial features of the Portevin-Le Chatelier plastic instabilities in single and polycrystals of Al-Mg alloys were investigated systematically, with special emphasis being put on the character of the statistical distributions of the stress drops. The effect of strain rate, temperature and the microstructural state of the alloy was studied experimentally. It was shown that an experimentally accessible quantity, the flow stress, governs to a large extent the observed correlation between the variation of the type of serrations and of the stress drop distributions. Computer simulations of the Portevin-Le Chatelier effect were carried out using a simple spatial coupling model. It was demonstrated that the salient features of the complex spatio-temporal behaviour observed experimentally for different microstructural states are adequately reproduced by the model. A comparison between the experimental data and the simulation results suggests that the spatial coupling stems from plastic strain incompatibilities.