sentence1: sentiment classification is a useful technique for analyzing subjective information in a large number of texts , and many studies have been conducted ( cite-p-15-3-1 ) .
sentence2: sentiment classification is a well studied problem ( cite-p-13-3-6 , cite-p-13-1-14 , cite-p-13-3-3 ) and in many domains users explicitly provide ratings for each aspect making automated means unnecessary .