A model represents selected features of a situation so that their relationships can be examined. Useful mod simplify com systems bec limited represe make impo patterns eas to ins while lea certain det outside th intended practical scope. A map of underground trains, for example, may show connections accurately while changing distances and directions. That choice can help a passenger plan a transfer, although it may mislead someone estimating a walk above ground. The omission is not necessarily a defect. Whether it matters depends on the question the model is being used to answer.

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