Hierarchical Search for Parsing

Both coarse-to-fine and A∗ parsing use simple grammars to guide search in complex ones. We compare the two approaches in a common, agenda-based framework, demonstrating the tradeoffs and relative strengths of each method. Overall, coarse-to-fine is much faster for moderate levels of search errors, but below a certain threshold A∗ is superior. In addition, we present the first experiments on hierarchical A∗ parsing, in which computation of heuristics is itself guided by meta-heuristics. Multi-level hierarchies are helpful in both approaches, but are more effective in the coarseto-fine case because of accumulated slack in A∗ heuristics.