The accuracy of weather forecasts is about 70% but if you say the weather tomorrow is going to be the same as today then you also have a 70% chance of success. In the same way the prediction of the behaviour of people with mental-health problems uses a mixture of assessments, questionnaires, the person's own predictions of their behaviour and the past history of their behaviour. Researchers at Harvard University compared the accuracy of people's forecasts of their own likelihood of engaging in self-injurious thoughts and behaviour (SITB) with forecasts based on their past history of such behaviour. Suicide is the third leading cause of death among adolescents in the U.S. and the study involved 64 adolescents with a past history of SITB. It found that their past histories of SITB was more accurate than their own predictions in forecasting future behaviour.
Janis, Irene Belle and Nock, Matthew K. - Behavioral forecasts do not improve the prediction of future behavior: a prospective study of self-injury Journal of Clinical Psychology October 2008, 64(10), 1164-1174