Are semantically related links effective for retrieval?

Why do links work? Link-based ranking algorithms are based on the often implicit assumption that linked documents are semantically related to each other, and that link information is therefore useful for retrieval. Although the benefits of link information are well researched, this underlying assumption on why link evidence works remains untested, and the main aim of this paper is to do exactly that. Specifically, we use Wikipedia because it has a dense link structure in combination with a large category structure, which allows for an independent measurement of the semantic relatedness of linked documents. Our main findings are that: 1) global, query-independent link evidence, is not affected by the semantic nature of the links, and 2) for local, query-dependent link evidence, the effectiveness of links increases as their semantic distance decreases. That is, we directly observe that links between semantically related pages are more effective for ad hoc retrieval than links between unrelated ones. These findings confirm and quantify the underlying assumption of existing link-based methods, which sheds further light on our understanding of the nature of link evidence. Such deeper understanding is instrumental for the development of novel link-based methods.