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CiteSpace was used to visualize the literature from 2003 to 2012 presented in Figure 1. The colors represent years, the squares are highly used terms (top), and the circular nodes are cited articles (bottom). Each colored circle represents the number of citations/uses during that year. Edge links between nodes represent co-occurrence (in the case of terms) and co-citation (in the case of articles), with the color representing the first year in which the connection was found.
