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.