The bottom-up approaches are the traditional ones, and they have the advantage that they require no assumptions on the overall structure of the document. On the other hand, bottom-up approaches require iterative segmentation and clustering, which can be time consuming. Top-down approaches are newer, and have the advantage that they parse the global structure of a document directly, thus eliminating the need to iteratively cluster together the possibly hundreds or even thousands of characters/symbols which appear on a document. They tend to be faster, but in order for them to operate robustly they typically require a number of assumptions to be made about on the layout of the document.