The second phase is to rank the candidates with machine learning methods.
some researchers Hulth, 2003;Medelyan et al., 2009;Gollapalli and Caragea, 2014) formulate the keyphrase extraction as a supervised classification problem, while others apply unsupervised approaches (Mihalcea and Tarau, 2004;Grineva et al., 2009;Liu et al., 2009Liu et al., , 2010Zhang et al., 2013;Bougouin et al., 2013Bougouin et al., , 2016 on this task.