Issues concerning the use of predictive and screening instruments include the difficulties posed by temporal changes in the base rates for the behaviour of interest, the possible advantages of using three rather than the customary two risk categories, and the desirability of specifying in advance the proportional sizes of the risk subgroups. A method is proposed for constructing and assessing tripartite risk scales, and a new scale for predicting the repetition of parasuicide, with the results of a prospective validation study, is reported.