The analysis of systems' dynamics lies on the collection and the description of events. In order to scale-up classical analysis methods, this report is interested in the reduction of descriptional complexity by aggregating events' properties. Shannon entropy appears to be an adequate complexity measure regarding the aggregation process. Some other informational measures are proposed to evaluate the qualities of aggregations: entropy gain, information loss, divergence, etc. These measures are applied to the evaluation of geographic aggregations in the context of news analysis. They allow determining which abstractions one should prefer depending on the task to perform.