Sample similarity in the multi-way sentiment detection setting has previously been considered by using Support Vector Machines (SVMs) in conjunction with a metric labeling metaalgorithm (Pang and Lee, 2005); by taking a semisupervised graph-based learning approach (Goldberg and Zhu, 2006); and by using "optimal stacks" of SVMs (Koppel and Schler, 2006).
each of these methods have shortcomings (Section 2).