After applying the two NLTK procedures mentioned above we ended up with over 500 million candidate phrases. Most of the candidate phrases were only encountered in a few documents which means these phrases do not contain enough information to be considered as scientific concepts. For this study we only considered phrases that have been found in at least 20 documents. After applying this constraint, the number of candidate phrases dropped to 1,451,511. For each of the candidate phrases we build its corresponding Citation Graph (CG). A CG is a directed graph where the nodes are documents, and the edges are citations of documents (Figure ). Figure . An example of a Citation Graph showing connections between nodes.