Confidence Lexicon: An Evidence-Based Approach for Communicating Eyewitness Confidence

We developed an evidence-based tool (a lexicon) which comprises verbal and numeric information for gathering and communicating eyewitness confidence. With two studies, we obtained interpretations of frequently used verbal confidence statements made by eyewitnesses that we used to derive the lexicon. We expected that a distinct set of phrases would span the range of confidence levels eyewitnesses express. Participants rated how well each of 11 percentage values (e.g., 0%, 60%, 100%) represented 13 verbal expressions of confidence previously obtained from participant-eyewitnesses (e.g., moderately confident) on a scale (0 = Not at all to 100 = Absolutely). Based on density plots (membership functions) for each phrase, we selected phrases that best represent the range of confidence levels eyewitnesses may wish to express. The lexicon comprises four phrases and three synonyms: Not very confident, moderately confident/somewhat confident, quite confident/pretty sure, very confident/confident. The phrases’ membership functions are distinct and together span 0-100% scale. There is broad agreement about the meaning of common verbal phrases for expressing confidence; we leveraged this agreement to produce a tool that can create greater common ground between eyewitnesses and triers of fact than simply asking the eyewitness to express their confidence in their own words.