A stochastic model of the chemical enrichment of metal-poor systems by core-collapse supernovae is used to study the scatter in relative elemental abundances in extremely metal-poor stars. The resulting scatter in abundance ratios is demonstrated to be crucially dependent on the as yet uncertain supernovae yields. The relatively small star-to-star scatter observed in many of these abundance ratios, e.g. by Cayrel et al. (2004), is tentatively explained by the averaging of a large number of contributing supernovae and by the cosmic selection effects favoring contributions from supernovae in a certain mass range for the most metal-poor stars. “Spurs”, very narrow sequences in abundance-ratio diagrams, may disclose a single-supernova origin of the elements of the stars on the sequence and would thus be an indication of an unmixed interstellar medium (ISM). Verification of the existence of such features, called single supernova sequences (SSSs), is challenging. This will require samples of several hundred stars with abundance ratios observed to accuracies of 0.05 dex or better.