CRF Baseline: We use a linear-chain CRF (Lafferty et al., 2001) of order 2 as our baseline, which is the state-of-the-art model for opinion target extraction (Pontiki et al., 2014).
the CRF generates (binary) feature functions of order 1 and 2; see (Cuong et al., 2014) for higher order CRFs.