Interpreting eyewitness confidence: Numeric, verbal, and graded verbal scales

In empirical research, eyewitnesses typically report their confidence numerically (e.g., "I'm 90% sure"). In contrast, in the field, lineup administrators typically ask witnesses to explain their confidence verbally, in the witness' own words (e.g., "I'm quite sure"). Across three studies, we explored how evaluators assess verbal confidence statements: both freely reported and reported using a graded verbal scale. Results showed wide variability in the interpretation of both kinds of confidence statements. Even when evaluating seemingly very strong statements of verbal confidence (e.g., "completely certain") participants did not necessarily translate these statements into the strongest levels of numeric confidence. Variability in the interpretation of verbal confidence was particularly pronounced for low confidence statements. Moreover, participants preferred to report their confidence numerically rather than verbally. These results indicate the importance of documenting confidence verbatim at the time of the lineup so that the meaning of the witness' original confidence statement is preserved.