1. A system for analyzing a sub-model separated from a full system model, said system model representing a system, said system comprising: means for defining the sub-model as a collection of entities in a visual medium; means for determining which of the entities in the sub-model are calculation entities and which are data entities; means for converting the calculation entities in the sub-model that depend on entities in the full model that are not included in the sub-model into temporary data entities; means for identifying output entities in the sub-model, where the output entities are calculation entities that do not have an output to another entity; and means for visually analyzing changes in the sub-model in response to performing the calculations for the calculation entities.