Speech sample size and test-retest stability of connected speech measures for adults with aphasia

The effect of speech sample size on the test-retest stability of two measures of connected speech--words per minute (WPM) and percent of words that are correct information units (Percent CIUs)--was evaluated. A standard set of 10 stimuli was used to elicit connected speech from 20 non-brain-damaged adults and 20 adults with aphasia. Each subject's responses to the 10 stimuli were transcribed and scored for WPM and Percent CIUs. Then each subject's responses to the 10 stimuli were randomly divided to produce smaller speech samples representing his or her responses to 1, 2, 3, 4, 5, and 7 stimuli. The test-retest stability of the WPM and Percent CIUs measures was then evaluated for each of the smaller sample sizes and for the complete 10-stimulus sample. For both groups, the test-retest stability of the two measures increased as sample size increased, with the greatest increases occurring as samples increased in size from those representing 1 stimulus to those representing 4 or 5 stimuli, with smaller increases in stability thereafter. In general, these results suggest that the best balance between high test-retest stability and the time and effort required to transcribe and score speech samples can be achieved with samples representing 4 or 5 stimuli (an average of 300 to 400 words for aphasic subjects), although this will vary across individuals.