The variation of 'goodbyes': A preliminary investigation for the purpose of forensic speaker comparison, using Dutch telephone conversations.

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In order to solve a crime, the police collect evidence and ask independent forensic experts to investigate it. When the evidence takes the form of a legally interepted telephone call, the police can ask a forensic speech expert to judge whether the voice in the recording is from the suspect, or whether it is more likely that the voice is actually someone else's. This is called a foresic speaker comparison (FSC). In order to formulate a judgement that is reliable and verifiable, the expert will determine for several features in the intercepted speaker's speech and voice how likely it is to find that feature in the suspect material, and how likely it is to find it in any other speaker. If a relatively rare feature occurs often in both the suspect material and the intercepted speaker's material, this will support the null hypothesis that the intercepted speaker is the suspect. Lexical elements that are independent from! the topi c of conversation are especially useful in FSC, because they are likely to occur in many speech recordings. One such lexical element are goodbyes: the greetings at the end of telephone calls. This Bachelor thesis investigates whether goodbyes in telephone conversations can be sufficiently rare or speaker-specific to be useful for forensic speaker comparison purposes. To that end, variation in the content and structure of goodbyes was investigated using a Dutch database of legally intercepted telephone calls. Four especially for this investigation determined features were used to describe the variation of the content and structure of goodbyes. Goodbyes could contain one or more keyword types, which are coarse representations of the core words of goodbyes, for example [dui]. Keyword tokens could be lengthened and replicated, and could occur on several positions in the goodbye. Statistical analyses showed gender differences in lengthening and replication of keywords: male spea! kers regu larly omit goodbyes and show a small preference for repetition of the keyword, while female speakers rarely omit goodbyes and show a strong tendency towards the lengthening of keywords. Moreover, crucially, goodbyes not only occur in many telephone conversations, but they can also be sufficiently rare and speaker-specific to be useful for forensic speaker comparison. This thesis has provided a first step towards the investigation of goodbyes for FSC.
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