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<Paper uid="W04-1203">
  <Title>Analysis of Link Grammar on Biomedical Dependency Corpus Targeted at Protein-Protein Interactions</Title>
  <Section position="4" start_page="0" end_page="15" type="intro">
    <SectionTitle>
2 Link Grammar and parser
</SectionTitle>
    <Paragraph position="0"> The Link Grammar and its parser represent an implementation of a dependency-based computationalgrammar. TheresultofLGanalysisfor a sentence is a labeled undirected simple graph, whosenodesrepresentthewordsofthesentence and whose edges and their labels express the grammatical relationships between the words.</Paragraph>
    <Paragraph position="1"> In LG terminology, the graph is called a linkage, and its edges are called links. The linkage must be planar (i.e., links must not cross) when drawn above the words of the sentence, and the labels of the links must satisfy the linking constraintsspecifledforeachwordinthegrammar. null A connected linkage is termed complete.</Paragraph>
    <Paragraph position="2">  the words binds to. The links joining these words form the interaction subgraph (drawn with solid lines).</Paragraph>
    <Paragraph position="3"> Due to the structural ambiguity of natural language, several linkages can typically be constructed for an input sentence. In such cases, the LG parser enumerates all linkages allowed by the grammar. Apost-processing step isthen  employedtoenforceanumberofadditionalconstraints. The number of linkages for some sentencescanbeveryhigh,makingpost-processing null and storage prohibitively expensive. This problem is addressed in the LG parser by deflning kmax, the maximal number of linkages to be post-processed. If the parsing algorithm produces more than kmax linkages, the output is reduced to kmax linkages by random sampling.</Paragraph>
    <Paragraph position="4"> Thelinkagesarethenorderedfrombesttoworst using heuristic goodness criteria.</Paragraph>
    <Paragraph position="5"> In order to be usable in practice, a parser is typically required to provide a partial analysis ofasentenceforwhichitcannotconstructafull analysis. If the LG parser cannot construct a complete linkage for a sentence, the connectedness requirement is relaxed so that some words do not belong to the linkage at all. The LG parser is also time-limited. If the full set of linkages cannot be constructed in a given time tmax, theparserentersa panic mode,inwhichit performsane-cientbutconsiderablyrestricted parse, resulting in reduced performance. The parameters tmax and kmax set the trade-ofi between the qualitative performance and the resource e-ciency of the parser.</Paragraph>
  </Section>
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