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BackgroundActuarial risk assessment instruments (ARAIs) estimate the probability that individuals will engage in future violence.AimsTo evaluate the ‘margins of error’ at the group and individual level for risk estimates made using ARAIs.MethodAn established statistical method was used to construct 95% CI for group and individual risk estimates made using two popular ARAIs.ResultsThe 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.ConclusionsThe 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.