The relationship between confidence and accuracy with verbal and verbal + numeric confidence scales

Police departments often use verbal confidence measures (highly confident, somewhat confident) with a small number of values, whereas psychologists measuring the confidence–accuracy relationship typically use numeric scales with a large range of values (20-point or 100-point scales). We compared verbal and verbal + numeric confidence scales for two different lineups, using either two or four levels of confidence. We found strong confidence–accuracy relationships that were unaffected by the nature of the scale at the highest level of confidence. High confidence corresponded to high accuracy with both two- and four-level scales, and the scale type (verbal only or verbal + numeric) did not matter. Police using a simple scale of “highly confident” and “somewhat confident” can, according to our results, rest assured that high confidence indicates high accuracy on a first identification from a lineup. In addition, our two lineups differed greatly in difficulty, yet the confidence–accuracy relationship was quite strong for both lineups, although somewhat lower for the more difficult lineup.