knowledge-based techniques can be mainly classified into two categories: dictionary-based and corpus-based approaches. dictionary-based approaches find opinion or emotion seed words in a dictionary and search for their synonyms and antonyms to expand the initial list of opinions or emotions. corpus-based approaches on the other hand, start with a seed list of opinion or emotion words, and expand the database by finding other words with context-specific characteristics in a large corpus. while corpus-based approaches take into account context, their performance still vary in different domains since a word in one domain can have a different orientation in another domain.