Violence risk assessment and risk communication: The effects of using actual cases, providing instruction, and employing probability versus frequencyformats

This article describes studies designed to inform policy makers and practit ioners about factors influencing the validity of violence risk assessment a nd risk communication. 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 mon ths after discharge from the hospital. They also judged whether the patient posed a high risk, medium risk, or low risk of harming someone after disch arge. Studies 1 and 2 replicated, with real case summaries as stimuli, the response-scale effects found by Slovic and Monahan (1995). Providing clinic ians with response scales allowing more discriminability among smaller prob abilities led patients to be judged as posing lower probabilities of commit ting harmful acts. This format effect was not eliminated by having clinicia ns judge relative frequencies rather than probabilities or by providing the m with instruction in how to make these types of judgements. In addition, f requency scales led to lower mean likelihood judgments than did probability scales, but, at any given level of likelihood a patient was judged as posi ng 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 frequenc y (e.g., 2 out of 10) led to much higher perceived risk than did communicat ing a comparable probability (e.g., 20%). The different reactions to probab ility and frequency formats appear to be attributable to the more frighteni ng images evoked by frequencies. Implications for risk assessment and risk communication are discussed.