There are many studies which handled news articles (Sakai et al., 2006), review documents (Ivanov and Tutubalina, 2014), financial documents (Leider and Schilder, 2010), daily reports (Kakimoto and Yamamoto, 2008), a failure database on the Web (Awano et al., 2012) and so on, as the target data.
these information sources are not usually instantaneous and exhaustive.