Assessing and Communicating Risk of Violence: Probabilities Differ from Relative Frequencies. P. Slovic and D. MacGregor, Decision Research, Eugene, OR; and J. Monahan, U. of VA School of Law, Charlottesville
Forensic psychologists and psychiatrists were shown case summaries of patients hospitalized with mental disorder and were asked to judge the likelihood that the patient would harm someone within six months after release from the hospital. They also judged whether the patient posed high risk, medium risk, or low risk of harming someone upon release. When likelihood of violence was judged on a scale of probabilities (10%, 20%, etc.) the mean probability was much higher than when the same patient was judged on a scale of relative frequencies (e.g. 10 out of 100 patients similar to this person would be expected to be violent, 20 out of 100, etc.). At a given level of likelihood, a patient was judged as posing higher risk if that likelihood was derived from a frequency scale (e.g. 10 out of 100) than if it was derived from a probability scale (e.g. 10%). Similarly, communicating a patient’s dangerousness as a relative frequency (e.g. 2 out of 10) led to much higher perceived risk than a comparable probability (e.g. 20%). The different reactions to probability and frequency formats appear to be attributable to the more frightening images evoked by frequencies. Implications for risk assessment and risk communication are discussed.
We thank the Decision Making, Risk, and Management Science Program of the NSF for financial support.
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