Anew scale for assessing perceptions of chance: A validation study. Medical Decision Making

Background. Clinicians and researchers often wish to know how patients perceive the likelihoods of health risks. Little work has been done to develop and validate scales and formats to measure perceptions of event probabilities, particularly low probabilities (i.e., <1%). Objective. To compare a new visual analog scale with three benchmarks in terms of validity and reliability. Design. Survey with retest after approximately two weeks. Respondents estimated the probabilities of six events with the new scale, which featured a "magnifying glass " to represent probabilities between 0 and 1 % on a log-arithmic scale. Participants estimated the same probabilities on three benchmarks: two linear visual analog scales (one labeled with words, one with numbers) and a "1 in x" scale. Subjects. 100 veterans and family members and 107 university faculty and students. Measures. For each scale, the authors assessed: 1) validity—the correlation between participants ’ direct rankings (i.e., numbering them from 1 to 6) and scale-derived rankings of the relative probabilities of six events; 2) test-retest reliability— the correlation of responses from test to retest two weeks later; 3) usability (missing/ incorrect responses, participant evaluation). Results. Both the magnifier and the tw