The most serious difficulty in developing a QA system is a lack of knowledge. In this paper, we first discuss three problems of developing a knowledge base by which a QA system answers How-type questions. Then, we propose a method of developing a knowledge base by using mails posted to a mailing list. Next, we describe a QA system which can answer How-type questions based on the knowledge base. Our system finds question mails which are similar to user's question and shows the answers to the user. The similarity between user's question and a question mail is calculated by matching of user's question and a significant sentence in the question mail. Finally, we show that mails posted to a mailing list can be used as a knowledge base by which a QA system answers How-type questions.
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