A Wordnet from the Ground Up

4.5. Hybrid Combinations 145Witschel (2005) applied a more radical decision-tree model with recursive upwardpropagation of meaning descriptions. The propagation only stops in the root, and thedescription of the upper nodes represents the description of descendants. A synset’ssemantic description is a set of LUs most similar to LUs from this synset. Similaritycalculation, following the distributional semantics model, is based on co-occurencesof LUs in corpus. Semantic descriptions of children nodes are recursively propagatedto parents and merged with their initial description. The resulting tree of semanticdescriptions is then used as a decision tree to assign new lemmas. We select a branch bythe highest similarity with a new lemma measured by the degree of matching betweendescriptions. Downward traversal stops in a node in which the mean of the similarityvalues with branches is greater than their variance. Evaluation was performed only ontwo subtrees taken from GermaNet: Moebel (furniture) (144 children) and Bauwerk(building) (902 children). The best accuracy of the exact classification was 14% and11% respectively, comparable to that achieved by Alfonseca and Manandhar (2002).Widdows (2003) represented LU meaning by the set of semantic neighbours – kmost similar LUs. The main idea for attaching a new lemma was to find a site in thehypernymy structure in which its semantic neighbours are concentrated. For semanticsimilarity calculation, each LU was first described by the co-occurrence, in a 15-wordtext window, with the selected 1000 most frequent one-word LUs. Parts of speech wereattached to words in the experiments that gave the best results. Similarity values werecomputed as in the Latent Semantic Analysis algorithm (Landauer and Dumais, 1997),cf Section 3.4.2. For the given LU and its first k semantic neighbours, a hypernym his chosen as its label (attachment point), such that it gives the highest sum over affinityscores between the subsequent neighbours and h. The affinity score is negative forneighbours which are not hyponyms of h, and positive otherwise, with higher valuefor neighbours closer to h.Evaluation was on the British National Corpus (BNC, 2007) and randomly selectedcommon nouns, 200 each from three frequency ranges: >1000, [500, 1000] and