1. A computer-implemented method of performing sentiment analysis, comprising: performing a first sentiment analysis on microblogging data based on a method using an opinion lexicon that includes non-domain-specific opinion terms, to generate first training data; training a classifier using the first training data; identifying a new opinion term in the microblogging data, by performing a statistical test on results of the first sentiment analysis on the microblogging data, wherein the new opinion term is domain-specific to the microblogging data and is not in the opinion lexicon; adding the new opinion term to the opinion lexicon to grow the opinion lexicon so that the opinion lexicon includes domain-specific opinion terms; identifying new microblogging data other than the microblogging data on which the first sentiment analysis has been performed, based on the opinion lexicon to which the new opinion term has been added; performing a second sentiment analysis on the new microblogging data using the classifier to generate second training data; and retraining the classifier as has been trained using the first training data, using the second training data to improve the classifier.