The effect of sample size on the assessment of stuttering severity.

The relationships between the length of the speech sample and the resulting disfluency data in 20 stuttering children who exhibited a wide range of disfluency levels were investigated. Specifically, the study examined whether the relative number of stuttering-like disfluencies (SLD) per 100 syllables, as well as the length of disfluencies (number of iterations per disfluent event), varied systematically across 4 consecutive, 300-syllable sections in the same speech sample. The difference in the number of SLD per 100 syllables between the early and later sections of the speech sample was statistically significant. In addition, the length of the speech sample had a critical influence on the identification of stuttering in children exhibiting relatively low levels of disfluency. Also, when a 20% difference in the number of SLD per 100 syllables was taken as a criterion, 50% of the children exhibited upward shifts in continuous speech samples that were longer than 300 syllables (i.e., 600, 900, and 1,200 syllables). Results indicated that, in general, group means for SLD grew larger as the sample size increased. The length of disfluent events did not significantly differ as the sample size increased; however, there were large differences for some children. Implications for clinicians and investigators are discussed.