We further note that iterative bootstrapping over a single distant supervision system is difficult, because state-of-the-art systems (Surdeanu et al., 2012; <cite>Hoffmann et al., 2011</cite>; Riedel et al., 2010; Mintz et al., 2009) , detect only few false negatives in the training data due to their high-precision low-recall features, which were originally proposed by Mintz et al. (2009) .