Approaches based on topic signatures are similar in spirit to our approach. A topic signature is a list
of topically related words (Agirre et al., 2000). There are many topic signature approaches (e.g., Agirre et
al., 2000; Zhou et al., 2007). Zhou et al. (2007) propose a Topic Signature Language Model that is used
to perform semantic smoothing to increase retrieval performance. They create topic signatures for each
concept defined in a domain specific ontology using a highly relevant document collection. The topic
signature terms are found by collocation. They assume unique concepts and consequently circumvent the
problem of word disambiguation. For general domains, where no ontology exists, they propose to use
multiword expressions as topic signatures. The multiword expressions contain context and are
consequently mostly unambiguous.