Disadvantages - Official statistics have to be used with caution, written documents such as diaries, letters or autobiographies could have been forged which could make them not genuine. Even if documents are genuine not all of the data may be true, some of the experiences or events that had been described could have been misinterpreted which could be due to the writer's opinions. Autobiographies tend to be one-sided and written for an audience, which is similar for letters that are written to a particular person as they are likely to be influenced by the writer's view of the recipient.