Left-corner Methods for Syntactic Modelingwith Universal Structural Constraints
Table 4.2: Token-level and sentence-level coverage results of left-corner oracles with depthre. Here, the right-hand numbers in each column are calculated from corpora that exclude all punctuation, e.g., 92% of tokens in Arabic are covered within a stack depth ≤3, while the number increases to 94.1 when punctuation is removed. Further, 57.6% of sentences (61.6% without punctuation) can be parsed within a maximum depthre of three, i.e., the maximum degree of center-embedding is at most two in 57.6% of sentences. Av. len. indicates the average number of words in a sentence.
['#Sents.', '#Sents.', '[BOLD] Arabic 3,043', '[BOLD] Arabic∗ 4,102', '[BOLD] Basque 3,523', '[BOLD] Bulgarian 13,221', '[BOLD] Catalan 15,125', '[BOLD] Chinese 57,647', '[BOLD] Czech 25,650']
We observe that results for Arabic are notably different from other languages. \newcitebuchholz-marsi:2006: CoNLL-X noted that the parse unit of the Arabic treebank is not a sentence but a paragraph in which every sentence is combined via a dummy root node. To remedy this inconsistency of annotation units, we prepared the modified treebank, which we denote as Arabic∗ in the figure, by treating each child tree of the root node as a new sentence. The results then are closer to other language treebanks, especially Danish, which indicates that the exceptional behavior of Arabic largely originates with the annotation variety. From this point, we review the results of Arabic∗ instead of the original Arabic treebank. Results showing that the token-level coverage reaches 99% in most languages with depthre≤3 indicate that the constructions with the degree three of center-embedding occurs rarely in natural language sentences. Overall, sentence-level coverage results are slightly decreased, but they are still very high, notably 95% – 99% with depthre≤3 for most languages.