In 1998 Fernaeus & Almkvist published a study of vrebal fluency that is included in the reference list. That study was based on 1 minute recordings. In later study, Cut the Coda, Per Östberg and I found that it is possible to make both types of verbal fluency tests shorter, and keep the diagnostic value.
Fernaeus, S-E., Östberg, P., & Hellström, Å. & Wahlund, L-O. (2008). Cut The Coda: Early Fluency Intervals Predict Diagnoses, Cortex, 44, 161-169.
I have not made any 2.5 minute study on verbal fluency.
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