Group versus individual statistical predictions

Lovely article in the Guardian (of all places!) by Christine Evans-Pughe on how making statistical predictions about individuals is exceedingly tricky. Reports work by Hart, Michie, and Cooke (2007).  Abstract of the latter:

BACKGROUND: Actuarial risk assessment instruments (ARAIs) estimate the probability that individuals will engage in future violence. AIMS: To evaluate the ‘margins of error’ at the group and individual level for risk estimates made using ARAIs. METHOD: An established statistical method was used to construct 95% CI for group and individual risk estimates made using two popular ARAIs. RESULTS: The 95% CI were large for risk estimates at the group level; at the individual level, they were so high as to render risk estimates virtually meaningless. CONCLUSIONS: The ARAIs cannot be used to estimate an individual’s risk for future violence with any reasonable degree of certainty and should be used with great caution or not at all. In theory, reasonably precise group estimates could be made using ARAIs if developers used very large construction samples and if the tests included few score categories with extreme risk estimates. 

Hart, S.D., Michie, C., & Cooke D.J. (2007) Precision of actuarial risk assessment instruments: Evaluating the ‘margins of error’ of group v. individual predictions of violence. British Journal of Psychiatry, 190, s60-s65.

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