“…While different authors deal with the matrix diagonal in disparate ways (Ahlgren et al, 2003), we took the three highest co-citations values and divided by two to approximate a document's co-citation frequency when matched with itself (White and Griffith, 1981). The science of citation-based mapping involves the spatial proximities of disciplines, fields, sub-fields and author oeuvres, and articles or documents (Small, 1999). Bibliographic studies based on citation and co-citation analysis typically rely on factor analysis, cluster analysis ,and multidimensional scaling analysis (Leydesdorff, 1987;McCain, 1990), each of which contributes additional insights by mapping the intellectual structure of the research fields.…”