Style variability in disfluency analysis for forensic speaker comparison

Disfluencies are a natural part of speech, often going unnoticed by both speaker and listener. Recent research on disfluency profiles (McDougall and Duckworth 2017, 2018) shows that they contain speaker-specific information which could be analysed and compared in forensic speaker comparison (FSC) casework. Since samples in FSC tend to be mismatched for speaking situation and style, the present study investigates the consistency of speakers' disfluency production across three forensically relevant tasks: a mock police interview, a paired conversation and a voicemail message. Disfluency production was found to differ significantly across tasks; in some cases, extreme within-speaker variation was observed. The results demonstrate that a speaker's disfluency behaviour is unlikely to remain consistent across different situations. However, it was found that some individuals who demonstrated unusual production of a particular type of disfluency showed relatively consistent production of that type across all three tasks. Consequently, we recommend that disfluency analysis is not used in FSC where there are marked differences in speaking style or situation, unless distinctive disfluency production is observed in a sample.