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168M. Blomgren, A.M. Goberman / Journal of Communication Disorders 41 (2008) 159–178Fig. 2. ANOVA interaction analysis: length of utterance (spatial variable) compared with speech rate stability(temporal variable). Data are mean percentage of stuttering for each speaking task (N = 22). Note: data points areordinal and are connected by lines only for illustrative purposes.speaking tasks in relation to the overall percentage of stuttered words (see previoussection). The overall mean percentage of words stuttered across the four tasks was 6.3%(S.D. = 6.9%) for the less severe group and 17.8% (S.D. = 14.7%) for the more severegroup. These groups were significantly different in their mean percentage of stuttering(t(86) = 4.72, p < .0001).For the more severe group, the mean percentage of stuttered words on the single words –habitual rate task was 8.5% (S.D. = 13.3%), the single words – variable rate task was 15.0%(S.D. = 9.7%), the initial words of the phrases – habitual rate task was 21.2% (S.D. = 15.5%),and the initial words of the phrases – variable rate task was 26.3% (S.D. = 12.9%). For the lesssevere group, the mean percentage of stuttered words on the single words – habitual rate taskwas 2.2% (S.D. = 2.9%), the single words – variable rate task was 7.1% (S.D. = 5.4%), theinitial words of the phrases – habitual rate task was 5.9% (S.D. = 4.7%), and the initial wordsof the phrases – variable rate task was 9.9% (S.D. = 9.9%). An ANOVA interaction plot foreach of the two severity groups is provided in Fig. 3.For the less severe group, there was no interaction between length of utterance andspeaking rate [F(1, 10) = 1.6, p = 0.23]. A significant main effect was found for rate [F(1,10) = 26.3, p = 0.0004], indicating that percentage of stuttering was significantly higher inboth variable rate tasks compared to both habitual rate tasks. A main effect for length ofutterance approached, but did not reach, significance [F(1, 10) = 4.7, p = 0.06]. For themore severe group, there was also no interaction between the variables [F(1, 10) = 0.77,p = 0.40]. However, there was a reversal of the main effect pattern compared to the mild
M. Blomgren, A.M. Goberman / Journal of Communication Disorders 41 (2008) 159–178 169Fig. 3. ANOVA interaction plots for the least severe stuttering participants (N = 11) and most severe stutteringparticipants (N = 11) showing the mean percentage of stuttering for each speaking task. Length of utterance(spatial variable) is compared with speech rate stability (temporal variable). Note: data points are ordinal and areconnected by lines only for illustrative purposes.group. In the severe group there was a significant main effect for length of utterance [F(1,10) = 21.0, p = 0.001], but not for rate [F(1, 10) = 4.2, p = 0.07]. Overall, the groupsdisplayed different performance patterns across the tasks. The more severe group had anincrease of stuttering frequency primarily in relation to increased length of utterance. Theless severe group had an increase in stuttering frequency primarily in relation to increasedspeech rate variability.4. DiscussionThe purpose of this study was to examine stuttering across a multidimensional hierarchyof speech performance tasks. A 2 2 speech performance matrix provided the approachfor structuring the tasks and studying the potential interaction between two performancevariables known to have an effect on stuttering: length of utterance and speech rate.For the stuttering speakers, a systematic increase in frequency of stuttering across thefour tasks/assessment conditions was identified. The findings clearly indicated that bothincreased spatial complexity (linguistic complexity as measured by length of utterance)and increased temporal complexity (motor complexity as measured by speech ratevariability) are associated with increased stuttering frequency. With respect to length ofutterance, the frequency of stuttering on the initial words of the phrases was significantlyhigher than the frequency of stuttering on single words (see Fig. 2). The increase in
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