Commonly used heuristics include (1) using a stop word list to remove non-keywords (e.g., Liu et al. (2009b) ) and (2) allowing words with certain partof-speech tags (e.g., nouns, adjectives, verbs) to be considered candidate keywords<cite> (Mihalcea and Tarau (2004)</cite> , Liu et al. (2009a) , Wan and Xiao (2008) ).