Length, grammatical complexity, and rate differences in stuttered and fluent conversational utterances of children who stutter

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KSEVIER

LENGTH, GRAMMATICAL COMPLEXITY, AND RATE DIFFERENCES IN STUTTERED AND FLUENT CONVERSATIONAL UTTERANCES OF CHILDREN WHO STUTTER KENNETH

J. L O G A N

and EDWARD

G. C O N T U R E

Syracuse University The purpose of this study was to assess length, grammatical complexity, and articulatory speaking rate differences in stuttered and perceptibly fluent conversational utterances produced by children who stutter. Subjects were 15 boys who stutter (mean age = 51.2 months, SD = 8.09 months), each of whom was audiotaped and videotaped while interacting with his mother during a 30-min play/conversation period. Twenty-five stuttered and 25 perceptibly fluent utterances from each subject's conversational speech sample were measured in terms of syllabic length, grammatical complexity, and articulatory speaking rate, and each utterance was categorized as "high" or "low" in length, grammatical complexity, and articulatory speaking rate relative to each subject's median for each of the three variables. Results indicated that syllabic length for stuttered utterances was significantly greater than that for perceptibly fluent utterances. Additional analysis showed that significantly more stuttered utterances were categorized as "high" in length and/or grammatical complexity, and that significantly more perceptibly fluent utterances were categorized as "low" in length and/or grammatical complexity. Results supported neither the notion that articulatory speaking rate differs between stuttered and perceptibly fluent utterances, nor the idea that articulatory speaking rate, when considered together with either utterance length or utterance grammatical complexity, determines whether an utterance will be stuttered or perceptibly fluent. Results are discussed in terms of current speech production models as well as current theory that suggests that speech fluency breakdowns are more likely to occur in utterances for which task demands (performance) exceed an individual's typical level of performance (capacity or ability). Several recent studies with both children who do not stutter (Bernstein Ratner and Sih, 1987; C o l b u r n and Mysak, 1982; G o r d o n and Luper, 1989; Gordon, Luper, and Peterson, 1986; H a y n e s and Hood, 1978; M c L a u g h l i n Address correspondence to Kenneth J. Logan, Communication Sciences and Disorders, Syracuse University, 805 South Crouse Avenue, Syracuse, NY 13244-2280. J. FLUENCYDISORD. 20(1995), 35~1 © 1995 by Elsevier Science Inc. 655 Avenue of the Americas, New York, NY 10010

0094-730X/95/$9.50 SSD1 0094-730X(94)00008-H

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and Cullinan, 1989; Pearl and Bernthal, 1980) and children who do stutter (Bernstein Rather and Sih, 1987; Gaines, Runyan, and Meyers, 1991; KadiHanifi and Howell, 1992; Weiss and Zebrowski, 1992) have found that increases in an utterance's grammatical complexity are associated with increases in speech disfluencies. The association between an utterance's length and its speech fluency, independent of grammatical complexity, has not been as extensively studied. Bernstein Ratner and Sih (1987) reported that, for imitated utterances six to 12 syllables in length, the positive correlation between utterance length and the percentage of stuttered utterances approached, but did not reach, statistical significance. However, Gaines et al. (1991) and Weiss and Zebrowski (1992), who examined this relationship in conversational speech, found that stuttered utterances were significantly longer than fluent utterances. It has been clearly shown in studies with adults who stutter (see Bloodstein, 1987, p. 388-391 for a review) that certain conditions that decrease speaking rate (e.g., delayed auditory feedback, rhythmic stimulation) also decrease stuttering frequency. However, there appears to be a complex relationship between speaking rate and speech fluency in persons who stutter. For instance, Stephenson-Opsal and Bernstein Ratner (1988) found that increases in speech fluency following indirect fluency intervention with two children who stuttered were associated with increases in their articulatory speaking rate (i.e., the number of syllables or words per utterance divided by the time taken to speak the utterance, minus any disfluencies or perceptible pauses within the utterance). In light of these findings, there seems to be some empirical support for the notion that the length, grammatical complexity, and articulatory speaking rate of an utterance relates to whether or not that utterance is spoken fluently. To date, however, there have been no published reports that have examined differences between stuttered and perceptibly fluent utterances by simultaneously measuring all three utterance variables (i.e., length, grammatical complexity, and articulatory speaking rate). Explicating length, grammatical complexity, and articulatory speaking rate differences between stuttered and perceptibly fluent utterances, and examining how these three variables might interact relative to speech fluency, seems important for at least two reasons. First, such findings may shed light upon recent speculation (e.g., Adams, 1990; Peters and Starkweather, 1990; Postma, Kolk, and Povel, 1990b; Starkweather and Gottwald, 1990) that certain utterances may be more "demanding" for speakers, particularly persons who stutter, to produce fluently. Second, such findings may have clinical relevance as several speech-language pathologists (e.g., Kelly and Conture, 1991; Ryan, 1979; Starkweather, Gottwald, and Halfond, 1990) have discussed the importance of modulating speech-language parameters such as utterance length, grammatical complexity, and/or speaking rate as part of their fluency therapy programs for young children who stutter.

DIFFERENCES IN STUTTERED AND FLUENT UTTERANCES

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Thus, the purpose of this study was two-fold: to examine length, grammatical complexity, and articulatory speaking rate differences between stuttered and perceptibly fluent conversational utterances produced by young children who stuttered, and to examine how these three variables might interact relative to speech fluency. The following questions were addressed: (1) Do the stuttered and perceptibly fluent conversational utterances of children who stutter differ in terms of length, grammatical complexity, or articulatory speaking rate? (2) Are certain types of conversational utterances (e.g., "long and rapidly articulated utterances" or "long and grammatically complex utterances") stuttered upon more often than other types of conversational utterances (e.g., "short and slowly articulated utterances" or "short and grammatically simple utterances")? (3) Is the difference between the number of stuttered and perceptibly fluent utterances within certain types of conversational utterances (e.g., "rapidly articulated") influenced by levels for the two remaining variables?

METHOD Subjects Subjects were 15 monolingual, Standard American English-speaking males who stuttered. Subjects ranged in age from 36 to 66 months, and their mean age was 51.2 months (SD = 8.09 months). All subjects were paid volunteers who were naive to the purposes and methods of the study. They were referred to the Syracuse University Gebbie Speech-Language-Hearing Clinics by their parents, other speech-language pathologists, or daycare, preschool, or school personnel. Criteria for subject inclusion were similar to those reported by Louko, Edwards, and Conture (1990) and Zebrowski and Conture (1989). Subjects had no known or reported hearing, neurological, developmental, academic, intellectual, or emotional problems, nor had any subject previously received speech-language therapy. Subjects' syntactic and morphologic development, as judged by their performance on Developmental Sentence Scoring (Lee, 1974), was within normal limits. Further, subjects who presented more than one age-inappropriate phonological process (see Grunwell, 1982; McReyholds and Elbert, 1981) or any unusual phonological processes (see Edwards and Shriberg, 1983; Stoel-Gammon and Dunn, 1985) were excluded from the study. In addition, each subject had to produce at least three or more within-word disfluencies (i.e., sound/syllable repetitions, audible or inaudible sound prolongations, and within-word pauses) and/or monosyllabic wholeword repetitions per 100 syllables of conversational speech, and at least one adult who knew the child well (e.g., parent, teacher) had to have expressed concern about the child's fluency.

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Table 1. Age, Time-Since-Onset, and Stuttering Severity Instrument (SSI) Data for the 15 Young Males who Stuttered with Group Means and Standard Deviations (SD) in Parentheses

Subject Number

Subjects' Chronological Age at Time of Data Collection (in months)

Subjects' Reported Age at Time of Onset (in months)

Time Between Reported Onset and Data Collection (in months)

Subjects' Total Overall Score on SSI

1 2 3a 4 5 6 7 8 9 10 11 12 13 14 15 M (SD)

36 42 46 47 47 48 48 48 52 52 54 58 62 62 66 51.20 (8.09)

29 30 * 36 22 25 39 42 36 42 33 42 48 36 36 35.43 (7.17)

7 12 * 11 25 23 9 6 6 10 21 16 14 26 30 16.14 (7.66)

17 18 18 22 18 17 22 22 10 20 21 21 20 15 17 18.53 (3.23)

a = parentscouldnot recallinformation. On average, 16.14 months (range = 6 - 3 0 months; SD = 7.65 months) had elapsed between the time that subjects had been first reported to stutter by their parents and the time of data collection for this study. Based on the results of the Stuttering Severity Instrument (SSI) (Riley, 1980) 10 of the 15 subjects were classified as presenting "moderate" stuttering, and five of the 15 subjects were classified as presenting "mild" stuttering. (See Table 1 for details pertaining to individual subjects). Six other young males who stuttered (M age = 45 months) were excluded from the study. Two of the excluded children failed to meet the previously described phonology criteria, whereas the other four excluded children failed to produce a sufficient number of stuttered and/or perceptibly fluent utterances (i.e., 25 of each) that met utterance selection criteria (described later) used for the study. Data Collection Data collection procedures, recording environment, and equipment were similar to those reported elsewhere (e.g., Conture and Kelly, 1991; Louko,

DIFFERENCES IN STUTTERED AND FLUENT UTTERANCES

39

Edwards, and Conture, 1990; Zebrowski and Conture, 1989). Each child was audiotaped and videotaped while interacting with his mother during one recording session that lasted approximately 30 min. Each mother/child pair was seated on opposite sides of a table, and a standard set of age-appropriate toys was placed between them. Each mother/child pair was instructed to play with the toys and interact as they would at home. Audiotape and videotape recordings were made for each 30-min mother/ child interaction. A lapel microphone (Sony, ECM-55), placed within 15 cm of each subject's mouth, was used to obtain the audio signal. All video recordings were made using a Panasonic WV 3500 color camera, set in a standard location 2 m from the subject. The camera was focused upon each subject's head, arms, and upper torso, and its output was channeled to a video switcher (Panasonic Model W J-3500). The output of a time code generator (Evertz Model 3600D) was time-locked to the camera's video images and the time code generator's output (displayed as hours:minutes:seconds:videoframes) was also channeled to the video switcher. The audio signal, video image, and the time code generator's time-locked output were simultaneously fed to a television monitor (Sony Trinitron) so that the time code appeared in the upper central portion of the television screen and provided a visually apparent running index (accurate to 33 msec) of each child's verbal and non-verbal behavior during the 30-min session. The audio, video, and time-code information were also simultaneously sent to a videotape recorder (Sony, Model BVU 200A), which recorded the audio, time code, and video (30 frames/s [60 video fields]) signals.

Utterance Selection Procedure Beginning at the 10-rain mark of each subject's videotape-recorded 30-rain conversation, verbatim transcripts of the first 50 utterances of each subject that met inclusion criteria for Lee's (1974) Developmental Sentence Scoring (DSS) procedure were made. That is, only utterances that were (1) fully intelligible, (2) spontaneously formulated, and (3) either contained a noun and verb in a subject-predicate relationship or were imperative statements, were included in the transcript. As per DSS guidelines, all multiclausal productions were classified as one utterance except in those instances where more than two independent clauses were joined by the conjunction and. Each corpus of 50 DSS-scorable utterances was analyzed and scored using the procedures described below to verify that each subject's performance on this measure of syntactic and morphologic production was within normal limits. More specifically, utterances that met the previous inclusion criteria were inspected for the presence of specific syntactic and morphologic structures within the following eight categories: indefinite pronouns/noun modifi-

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K.J. LOGAN AND E. G. CONTURE

ers, personal pronouns, main verbs, secondary verbs, negatives, conjunctions, interrogative reversals, and Wh-questions. Each DSS-scorable structure within each of the 50 utterances that a subject used appropriately (i.e., the adult form of Standard American dialect) was assigned a specified numerical value, weighted to reflect its relative developmental difficulty. Numerical weights for the linguistic structures scored within each utterance were then summed, and an extra point was assigned to the utterance if the entire utterance was grammatically correct (i.e., the adult form of Standard American dialect). To facilitate the speed and accuracy of the DSS procedure, a preliminary analysis of each subject's transcript was performed using the computer software package Computerized Profiling (Long and Fey, 1989). The first author then reviewed the preliminary computer analyses, making appropriate adjustments to sentence scoring when necessary. Each utterance within each subject's corpus of 50 DSS-scorable utterances was then assessed for the presence of within-word disfluencies (i.e., sound/syllable repetitions, audible or inaudible sound prolongations, and within-word pauses), monosyllabic whole-word repetitions, and between-word disfluencies (i.e., multisyllabic whole-word repetitions, phrase repetitions, interjections, revisions). (See Conture (1990) for examples of each disfluency type.) Based on this assessment, each utterance was then classified as either (1)perceptibly fluent, (2) stuttered, or (3) other disfluent. Perceptibly fluent utterances were defined as those utterances that contained no instances of speech disfluency. Stuttered utterances were defined as those utterances that contained only withinword disfluencies and/or monosyllabic whole-word repetitions. Other disfluent utterances were defined as those utterances that contained one or more betweenword disfluencies. Because the purpose of this study was to compare stuttered to perceptibly fluent utterances, the other disfluent utterances were excluded from further analysis. As an average of 7.07 (SD = 3.49) other disfluent utterances (utterances that were subsequently excluded from further analysis) were produced within each of the 15 subject's initial corpus of 50 DSS-scorable utterances, it was necessary to resume transcription of each subject's videotaped conversations beyond the initial 50 utterances. Transcription of each subject's videotaped conversation proceeded until 25 perceptibly fluent and 25 stuttered DSS-scorable utterances were identified for each of the 15 subjects. All subsequent measurements of utterance length, grammatical complexity, and articulatory speaking rate were based on each subject's corpus of 25 perceptibly fluent and 25 stuttered DSS-scorable utterances.

Measurements of Utterance Length, Grammatical Complexity, and Articulatory Speaking Rate Length measurements were made by counting the number of syllables contained within perceptibly fluent segments of each subject's 25 perceptibly

DIFFERENCES IN STUTTERED AND FLUENT UTTERANCES

41

fluent and 25 stuttered utterances. Syllables were selected over words as a measurement unit for utterance length for two reasons. First, it was thought that syllables could be identified more reliably than words. Second, because words vary widely in their length, it was thought that syllables, which have less absolute variation than words, would provide a more accurate reflection of utterance length from one utterance to the next. Indirect support for the latter point comes from a study by Brundage and Bernstein Ratner (1989), who found that measuring length in syllabic units yields a more precise index of stuttering frequency than does measuring length in word units. Grammatical complexity measurements were made by calculating the DSS score (described above) for each of the 50 utterances. Utterances with higher DSS scores were assumed to be more grammatically complex than those with lower DSS scores. Articulatory speaking rate measurements (in syll/min) were based upon procedures similar to those described elsewhere (e.g., Costello and Ingham, 1984; Meyers and Freeman, 1985; Perkins, 1975); that is, the number of syllables per utterance was divided by the total time in milliseconds from the beginning to the end of each DSS-scorable utterance less the duration of any stutterings (i.e., within-word speech disfluencies and monosyllabic wholeword repetitions), ~ stuttering-related pauses (i.e., inaudible sound prolongations), or unusually long pauses (i.e., pauses that exceeded 10 videoframes [approximately 330 ms]). All beginning and ending times, pause boundaries, and disfluency boundaries for each subject's 25 perceptibly fluent and 25 stuttered utterances were measured on a videoframe-by-videoframe basis. accurate to 16.5 msec, using the videotape's visually apparent time-code.

Intrajudge and lnterjudge Measurement Reliability Fifteen utterances, one randomly selected from each of the 15 subjects, were selected to assess intrajudge (author) and interjudge (author versus a certified speech-language pathologist) measurement reliability for speech fluency, utterance length, DSS, and articulatory speaking rate measurements. Measurement agreement indexes (i.e., agreements divided by agreements plus disagreements, multiplied by 100) for the 15 samples indicated intrajudge/ interjudge agreements of 100%/100% for judging utterance speech fluency. Because utterance length (syllables), DSS scores (in the present study these scores ranged from 0 to 44), and articulatory speaking rate (syll/min) are continuous rather than categorical measures, measurement reliability

Jlt will be recalled that utterances containing between-word disfluencies (i.e., other disfluent utterances~ were excluded from further consideration in this study; therefore, only the duration of within-word disfluencies and monosyllabic whole-word repetitions needed to be removed from the total duration of a stuttered utterance when calculating articulatory speaking rate.

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Table 2. Individual and Group Means (Standard Deviations) for Utterance Length, Utterance Grammatical Complexity, and Utterance Articulatory Speaking Rate are Displayed for 25 Perceptibly Fluent (F) and 25 Stuttered (S) Utterances per Each of the 15 Young Males who Stuttered

Utterance Length (# syll/utt) Subject 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Group M (SD)

Utterance Grammatical Complexity (DSS score)

Utterance Articulatory Speaking Rate (syll/min)

F

S

F

S

F

S

4.64 (1.82) 4.64 (1.98) 4.16 (1.46) 5.16 (1.65) 4.68 (2.10) 5.00 (2.06) 4.56 (1.76) 5.16 (2.90) 4.84 (1.84) 4.56 (1.45) 4.00 (1.63) 5.24 (1.88) 5.56 (2.24) 7.64 (2.87) 8.68 (4.04)

5.00 (1.68) 5.64 (2.80) 5.28 (2.84) 6.16 (3.02) 7.16 (2.63) 6.36 (2.04) 5.52 (2.77) 7.32 (3.19) 5.56 (2.31) 5.40 (2.12) 5.76 (2.42) 6.64 (2.56) 7.04 (2.62) 8.76 (3.32) 10.08 (5.24)

6.36 (3.43) 8.32 (6.11) 5.56 (3.69) 7.04 (4.27) 6.32 (4.17) 6.20 (3.76) 6.12 (3.93) 8.00 (5.20) 5.64 (3.13) 7.04 (3.84) 5.52 (3.31) 6.36 (3.39) 6.48 (4.16) 8.72 (6.16) 9.68 (7.67)

6.12 (3.27) 7.88 (5.67) 7.32 (5.11) 7.76 (4.39) 12.00 (7.65) 7.96 (4.58) 6.84 (5.41) 14.68 (9.52) 6.16 (3.25) 5.40 (3.91) 6.96 (4.28) 8.92 (4.06) 10.24 (6.70) 7.72 (4.61 ) 11.60 (8.26)

218.11 (62.55) 241.34 (67.68) 193.08 (62.72) 176.57 (41.97) 251.39 (86.86) 230.70 (74.40) 223.71 (64.81) 252.90 (78.17) 245.19 (108.86) 176.70 (47.67) 235.25 (65.24) 203.41 (64.51 ) 269.58 (59.47) 261.37 (45.94) 277.31 (71.63)

215.07 (47.07) 226.64 (50.65) 199.67 (43.36) 183.86 (53.19) 250.20 (61.83) 193.70 (41.49) 233.34 (54.75) 251.35 (52.14) 236.35 (63.38) 172.90 (52.74) 221.64 (73.43) 189.21 (45.58) 269.46 (72.01) 248.76 (45.19) 242.57 (57.44)

5.24 (2.50)

6.50 (3.13)

6.89 (4.66)

8.59 (6.06)

230.46 (74.13)

222.32 (60.86)

DIFFERENCES IN STUTTERED AND FLUENT UTTERANCES

43

scores for these variables were expressed in mean difference scores rather than percentage of agreement indices. Mean intrajudge difference scores were as follows: utterance length (M = 0.00, SD = 0.00), DSS score (M = 0.33, SD = 1.29), and articulatory speaking rate (M = 6.13, SD = 9.20). Mean interjudge difference scores were as follows: utterance length (M = 0.00, SD = 0.00), DSS score (M = 0.20, SD = 1.47) and articulatory speaking rate (M = 12.53, SD = 13.17).

RESULTS Individual Subject Performance Table 2 lists means and standard deviations for utterance length, grammatical complexity (DSS scores), and articulatory speaking rate data derived from the 25 perceptibly fluent and 25 stuttered utterances produced by each of the 15 subjects. (Group means and standard deviations are presented at the bottom of Table 2.) Inspection of subjects' data showed that mean utterance lengths were greater in stuttered utterances than in perceptibly fluent utterances for all 15 subjects; mean utterance grammatical complexity scores were greater in stuttered utterances than in perceptibly fluent utterances for 11 of the 15 subjects; and mean articulatory speaking rates were greater in perceptibly fluent utterances than in stuttered utterances for 12 of the 15 subjects.

Differences Between Length, Grammatical Complexity, and Articulatory Speaking Rate of Stuttered Vs. Fluent Utterances Figure I depicts the group (N = 15) means and standard deviations associated with utterance length, grammatical complexity, and articulatory speaking rate for perceptibly fluent and stuttered utterances. Three paired-sample t-tests were performed, with Type I error tolerance per test = 0.017 (overall alpha = 0.05). Results indicated that stuttered utterances were significantly (t = 9.05; p < 0.001) longer than perceptibly fluent utterances. Although the observed value for utterance grammatical complexity (i.e., mean DSS scores) was greater for stuttered utterances than for perceptibly fluent utterances, this difference was not statistically significant (t = 2.69; p = 0.02). Similarly, the observed value for articulatory speaking rate was greater for perceptibly fluent utterances than for stuttered utterances; however, this difference was not statistically significantly (t = 2.29; p = 0.04).

Differences Between Number of Stuttered and Perceptibly Fluent Utterances Across Utterance Categories A median split procedure was used to further examine differences between stuttered and perceptibly fluent utterances. That is, each of the 15 subjects'

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300

STUTTERED UTTERANCES 2 O0

I

FLUENT UTTERANCES

MEAN SPEAKING RATE ( S Y L L / M I N) I00

Figure 1. A three-dimensional representation of the average syllabic lengths, articulatory speaking rates (syll/min), and DSS scores for 375 fluent and 375 stuttered utterances produced by young males who stuttered (N = 15). Mean articulatory speaking rates are represented by the tops of the two thick lines, whereas mean utterance lengths and mean DSS scores are represented by the bottom of each thick line, The thin vertical lines represent + or - one standard deviation for utterance articulatory speaking rate. The thin, crossed lines at the bottom of the thick line labeled "Fluent Utterances" and at the bottom of the thick line labeled "Stuttered Utterances" represent either + or - one standard deviation for utterance lengths or DSS scores. 50 utterances was categorized as being "high" (i.e., above the median) or "low" (i.e., below the median) in length, "high" or "low" in grammatical complexity, and "high" or "low" in articulatory speaking rate, relative to each subject's median for each of the three variables, e For each of the 15 subjects, this categorization process resulted in three 2 x 2 tabulations (i.e., length x grammatical complexity; length x articulatory speaking rate; and grammatical complexity x articulatory speaking rate) for stuttered and for

2Individual as opposed to group, medians were selected as a reference for the median split procedure to provide a more precise estimate of how the length, grammatical complexity, and articulatory speaking rate of each utterance produced by each subject compared to his typical performance.

DIFFERENCES IN STUTTERED AND FLUENT UTTERANCES

12 --

45

*(p=O.OO8)

10

*(p
E

Z Z~

HiLen HiDSS

LoLen HiDSS

I

l l Fluent Utterances

HiLen LoDSS

LoLen LoDSS

[ ] Stuttered Utterances ]

Figure 2. Means (bars) and standard deviations (thin vertical lines) of perceptibly fluent and stuttered utterances produced by the 15 young males who stuttered during four conversational conditions. Utterances were either high (Hi) or low (Lo) in length (Len) or grammatical complexity (DSS) relative to subjects' median values for respective variables. Asterisks (*) indicate significant differences for the four pairedsample t-tests. Type I error tolerance per test = 0.0125 (overall alpha = 0.05).

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perceptibly fluent utterances. 3 Averaged across the 15 subjects, the results of these 2 x 2 tabulations are illustrated in Figures 2-4, each of which shows the mean number (and standard deviation) of stuttered or perceptibly fluent utterances associated with each of the four possible outcomes (e.g., "highlow," "low-high,", "high-high," or " l o w - l o w " ) that resulted when combining any two of the three variables (i.e., length, grammatical complexity, or articulatory speaking rate). Paired-sample t-tests were used to analyze differences between the number of stuttered and perceptibly fluent utterances falling into the various categories (e.g., number of stuttered versus perceptibly fluent utterances that were "High Rate - Low DSS").

Utterance Length and Grammatical Complexity. Figure 2 shows the mean number of stuttered versus perceptibly fluent utterances associated with the four possible outcomes for the combinations of utterance length and grammatical complexity. Results of four paired-sample t-tests with Type I error tolerance per test = 0.0125 (overall alpha = 0.05) indicated that the observed difference in the frequency of occurrence of stuttered and perceptibly fluent utterances achieved statistical significance for the following categories: "High Length - High DSS" (t = 3.12; p = 0.008) and "Low Length Low DSS" (t = -4.87; p < 0.001). Observed differences in the frequency of occurrence of stuttered and perceptibly fluent utterances did not achieve statistical significance in the following categories: "High Length - Low DSS" (t = 2.41; p = 0.03) and "High DSS - Low Length" (t = 0.01; p = 0.92). Utterance Length and Articulatory Speaking Rate. Figure 3 shows the mean number (and standard deviation) of stuttered versus perceptibly fluent utterances associated with the four possible outcomes for the combinations of utterance length and articulatory speaking rate. Results of four pairedsample t-tests with Type I error tolerance per test = 0.0125 (overall alpha = 0.05) indicated that the observed difference in the frequency of occurrence of stuttered and perceptibly fluent utterances achieved statistical significance for the following categories: "High Length - Low Rate" (t = 4.88; p < 0.001), "Low Length - High Rate" (t = -3.87; p = 0.002), and "Low Length - Low Rate" (t = -4.04; p = 0.001). Although more stuttered utterances than perceptibly fluent utterances were observed in the "High Length - High Rate" category, this difference was not statistically significant (t = 1.92; p = 0.08).

3Although it may have been preferable to use three-way (e.g., "High Length - High DSS High Rate") tabulations to compare stuttered and perceptibly fluent utterances, several subjects did not produce certain types of utterances (e.g., "Low Length - High DSS - High Rate"); that is, they produced no data to fill.in specific cells for the three-way tabulation. Thus, the authors were restricted in this preliminary study to assessing the three variables two at a time.

DIFFERENCES IN STUTTERED AND FLUENT UTTERANCES

47

*(p<0.001)

(p=O.OS)

8

*(p=O.O01)

6

*(p=O.O02) 0 e'~

3

_

HiLen HiRate

__+_

LoLen HiRate Fluent Utterances

HiLen LoRate

LoLen LoRate

[ ] Stuttered Utterances /

J

Figure 3. Means (bars) and standard deviations (thin vertical lines) of perceptibly fluent and stuttered utterances produced by the 15 young males who stuttered during four conversational conditions. Utterances were either high (Hi) or low (Lo) in length (Len) or articulatory speaking rate (Rate) relative to subjects' median values for respective variables. Asterisks (*) indicate significant differences for the four pairedsample t-tests. Type I error tolerance per test = 0.0125 (overall alpha = 0.05).

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(p=O.12)

*(p=0.002)

(D=0.20)

(p=O.07)

0 ~

4

~z

HiDSS HiRate

LoDSS HiRate

l I I Fluent Utterances

HiDSS LoRate

LoDSS LoRate

~ Stuttered Utterances ]

Figure 4. Means (bars) and standard deviations (thin vertical lines) of perceptibly fluent and stuttered utterances produced by the 15 young males who stuttered during four conversational conditions. Utterances were either high (Hi) or low (Lo) in grammatical complexity (DSS) or articulatory speaking rate (Rate) relative to subjects' median values for respective variables. Asterisks (*) indicate significant differences for the four paired-sample t-tests. Type I error tolerance per test = 0.0125 (overall alpha = 0.05).

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Grammatical Complexity and Articulatory Speaking Rate. Figure 4 shows the mean number (and standard deviation) of stuttered versus perceptibly fluent utterances associated with the four possible outcomes for the combinations of grammatical complexity and articulatory speaking rate. Results of four paired-sample t-tests with Type I error tolerance per test = 0.0125 (overall alpha = 0.05) showed that the observed difference in the frequency of occurrence of stuttered and perceptibly fluent utterances achieved statistical significance in only one category: "High DSS - Low Rate" (t = 3.90; p = 0.002). Observed differences in the frequency of occurrence of stuttered and perceptibly fluent utterances did not achieve statistical significance for the following categories: "High DSS - High Rate" (t = 1.68; p = 0.12), "Low DSS - High Rate" (t = -1.94; p = 0.07), and "Low DSS - Low Rate" (t = -1.36; p = 0.20). In summary, the observed number of stuttered utterances was significantly greater than the observed number of perceptibly fluent utterances in the following utterance categories: "High Length - High DSS," "High Length Low Rate," and "High DSS - Low Rate." Conversely, the observed number of perceptibly fluent utterances was significantly greater than the observed number of stuttered utterances in the following utterance categories: "Low Length - Low DSS," "Low Length - High Rate," and "Low Length - Low Rate." No significant differences were observed between the frequency of stuttered and perceptibly fluent utterances in any of the other two-variable comparisons.

Relationships Among Variables Data from the median split procedure were also used to examine whether the difference in the observed number of stuttered and perceptibly fluent conversational utterances for any one level (e.g., "High") of one variable (e.g., length) differed between levels (i.e., "High" or "Low") of the remaining two variables (e.g., grammatical complexity, articulatory speaking rate). Three series of paired sample t-tests (one series each for length, grammatical complexity, and articulatory speaking rate) were used to test this notion. Type I error tolerance for each test within each series of paired sample t-tests equaled 0.0125 (overall alpha = 0.05).

Effects of DSS and Rate upon Length. The mean difference (M = 0.07) between stuttered and perceptibly fluent utterances categorized as "Low Length - High DSS" was significantly different (t = -3.25, p = 0.006) from the mean difference (M = -4.87) between stuttered and perceptibly fluent utterances categorized as "Low Length - Low DSS." No other statistically significant differences were found between the number of observed stuttered and perceptibly fluent utterances for high or low levels of length when either

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articulatory speaking rate or grammatical complexity was high or low. In essence, results suggest that for utterances categorized as "Low Length," the difference in the number of stuttered and perceptibly fluent utterances was dependent only upon whether DSS level was high or low.

Effects of Length and Rate upon DSS. The mean difference (M = 1.74) between stuttered and perceptibly fluent utterances categorized as "High Length - Low DSS" was significantly different (t = 4.68, p < 0.001) from the mean difference (M = -4.87) between stuttered and perceptibly fluent utterances categorized as "Low Length - Low DSS." No other statistically significant differences were found between the number of observed perceptibly fluent and stuttered utterances for high or low levels of DSS when either articulatory speaking rate or length was high or low. In essence, results suggest that for utterances classified as "Low DSS," the difference in the number of stuttered and perceptibly fluent utterances was dependent only upon whether utterance length was high or low. Effects of Length and Grammatical Complexity upon Rate. The mean difference (M = 1.10) between stuttered and perceptibly fluent utterances categorized as "High Length - High Rate" was significantly different (t = 3.51; p = 0.003) from the mean difference (M = 3.80) between stuttered and perceptibly fluent utterance categorized as "Low Length - High Rate." Likewise, the mean difference (M = -2.10) between stuttered and perceptibly fluent utterances categorized as "High Length - Low Rate" was significantly different (t = -6.40, p < 0.001) from the mean difference (M = 2.70) between stuttered and perceptibly fluent utterances categorized as "Low Length - Low Rate." No other statistically significant differences were found between differences in observed stuttered and perceptibly fluent utterances for high or low levels of articulatory speaking rate when either length or grammatical complexity was high or low. In essence, results suggest that for utterances classified as either "High Rate" or "Low Rate," the difference in the number of stuttered and perceptibly fluent utterances was dependent upon whether utterance length was high or low. DISCUSSION

Utterance Length In the present study, stuttered conversational utterances produced by the 15 young males who stuttered were significantly longer than the perceptibly fluent conversational utterances they produced. In addition, significantly more stuttered utterances than perceptibly fluent utterances were noted in two of the four "High Length" utterance categories, and significantly more perceptibly fluent utterances than stuttered utterances were noted in three of

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the four "Low Length" utterance categories. Further, findings from the present study appear to provide support for the idea that length differences between stuttered and perceptibly fluent conversational utterances a r e independent of utterance articulatory speaking rate, but a r e n o t independent of utterance grammatical complexity. The finding that stuttered conversational utterances are significantly longer than perceptibly fluent conversational utterances is consistent with results from studies by Gaines et al. (199l) and Weiss and Zebrowski (1992), each of whom also studied differences between the lengths of stuttered and fluent conversational utterances. Present findings are not, however, consistent with those of Bernstein Ratner and Sih (1987) who reported only a nearly significant positive correlation between the syllabic length of immediately imitated utterances and the percent of utterances within each length category that contained at least one stuttering. Differences in speaking tasks may account for some of the difference between Bernstein Ratner and Sih's findings and those of the present study. That is, as Masterson and Kamhi (1992) have suggested, children's speech and language behavior may differ substantially between imitative and conversational speaking tasks.

Utterance Grammatical Complexity Results from the present study appear to provide p a r t i a l support for the notion that stuttered conversational utterances are more grammatically complex than perceptibly fluent conversational utterances. That is, though subjects' mean DSS scores for stuttered utterances were not significantly greater than those for perceptibly fluent utterances, there were significantly more stuttered utterances than perceptibly fluent utterances for two of the four categories of "High DSS" utterances, and there were significantly more perceptibly fluent utterances than stuttered utterances for one of the four categories of "Low DSS" utterances. Findings from the present study also appear to provide support for the idea that differences in the ratio of stuttered to perceptibly fluent conversational utterances for levels of grammatical complexity a r e independent of utterance articulatory speaking rate, but a r e n o t independent of utterance length. The present finding of no significance difference in grammatical complexity (i.e., DSS scores) between stuttered and perceptibly fluent conversational utterances was unexpected, particularly in light of findings from two recent studies (Gaines et al., 1991; Weiss and Zebrowski, 1992) that also used DSS to measure the grammatical complexity of stuttered and fluent conversational utterances. In both the Gaines et al., and Weiss and Zebrowski studies, the stuttered conversational utterances of children who stuttered were significantly more grammatically complex than their fluent conversational utterances.

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One possible explanation for the differences in findings between the present study and that by Gaines et al. (1991) relates to the issue of stuttering severity. That is, based on their SSI ratings, the 15 subjects in the present study (5 "mild," l0 "moderate") displayed less severe stuttering than did Gaines et al.'s 12 subjects (5 "moderate," 7 "severe"). Perhaps the speech disfluencies of children with "severe" stuttering behavior are more related to increases in grammatical complexity than those of children with "mild" stuttering behavior. This is not an entirely sufficient explanation, however, because Weiss and Zebrowski (1992), using subjects whose stuttering severity ratings were more similar to those in the present study, also found that stuttered conversational utterances were significantly more grammatically complex than fluent conversational utterances. On the other hand, subjects in Weiss and Zebrowski's study were older and their ages were more variable than subjects in the present study. Perhaps, differences in the grammatical complexity of stuttered and fluent conversational utterances are influenced to a greater extent by variations in stuttering severity and chronological and developmental age than are differences in the length of these utterances. Present findings also differ somewhat from Bernstein Rather and Sih's (1987) report of a strong positive correlation in children who stutter between the ranked grammatical complexity of a sentence and its tendency to contain at least one stuttering when imitated. Sentence types considered by Bernstein Ratner and Sih to be most difficult (i.e., sentences containing right-, center-, and left-embedded relative clauses) were most likely to contain at least one stuttering. However, such relative clauses appear infrequently in the casual conversation of school-aged children (Scott, 1988)--1et alone that of preschool-aged children. Hunt (cited in Scott, 1988) estimated that first-graders use approximately one relative clause per 100 terminable-units (i.e., sentences) of discourse, lndeed, inspection of 150 utterances produced by the three subjects with the highest mean DSS scores in the present study revealed only one, left-embedded, relative clause. Thus, it is uncertain whether some of the syntactic structures employed by Bernstein Ratner and Sih to assess the relationship between grammatical complexity and speech fluency are most representative of those exhibited by typical preschool-aged children during conversational speech.

Utterance Articulatory Speaking Rate Results from the present study fail to provide support for the idea that the articulatory speaking rates of stuttered and perceptibly fluent conversational utterances differ significantly. Indeed, for 13 of the 15 subjects in this study, the mean articulatory speaking rates for stuttered and perceptibly fluent utterances were quite comparable (i.e., within 15 syll/min). The remaining two subjects showed, contrary to what might be expected, mean articulatory

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speaking rates that were considerably faster in fluent utterances than they were in stuttered utterances. Finding significantly more perceptibly fluent than stuttered utterances in both "High Rate" (i.e., "Low Length - High Rate") and "Low Rate" (i.e., "Low Length - Low Rate") categories suggests that the fluency of an utterance has more do with the length of an utterance than it has do with the level of articulatory speaking rate. Likewise, finding significantly more stuttered utterances than perceptibly fluent utterances in other "Low Rate" categories (i.e., "High Length - Low Rate"; "High DSS - Low Rate"), suggests that the higher number of stuttered utterances in these categories has more do with the fact that utterance length and grammatical complexity were "High," than it has do with the level of articulatory speaking rate. Thus, taken together, these findings suggest that articulatory speaking rate, at least at the utterance level, may not be as robust as explanatory variable relative to the presence or absence of stuttering as it is often thought to be. Present findings, which fail to provide support for the idea that the articulatory speaking rates of stuttered and perceptibly fluent conversational utterances differ significantly, are consistent with those from a recent study by Kalinowski et al. (1993) in which subjects who stuttered were able to produce perceptibly fluent speech while speaking under delayed auditory feedback at both fast and more typical speaking rates. More generally, articulatory speaking rate data from the present study are in agreement with normative data reported by Walker et al. (1992) for children who did not stutter who were within 2 months of either their third or fifth birthday. It is interesting to note, however, that the articulatory speaking rates for some of the younger children in the present study were more than one standard deviation below Walker et al.'s mean for 3-year-old children who did not stutter. Differences in articulatory speaking rates for these subjects may reflect differences between the speaking contexts used in the present study (conversational speech samples) versus Walker et al.'s study (narrative speech samples) as well as differences in the nature of talker groups used in the two studies. The group mean articulatory speaking rate (226 spm) for children in the present study was somewhat faster than the group mean articulatory speaking rate (200 spm) reported by Kelly and Conture (1992) for children who stuttered. Differences in mean articulatory speaking rates between the two studies may be attributable to disparities in subjects' ages. That is, the three children with the fastest mean articulatory speaking rates in the present study were each more than 5 years old, whereas the oldest subject in Kelly and Conture's study was 4:8.

Theoretical and Clinical Implications These data partially support the notion discussed previously by others (e.g., Adams, 1990; Andrews and Neilson, 1981; Kelly and Conture, 1992; Peters

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and Starkweather, 1990; Postma, Kolk, and Povel, 1990b; Starkweather, 1987; Starkweather and Gottwald, 1990) that speech fluency breakdowns are more likely to occur in utterances for which task demands (performance) exceed an individual's typical level of performance (capacity or ability). Based on data in the present study, this notion is best supported when considering an utterance's length, partially supported when considering an utterance's grammatical complexity, and not supported when considering an utterance's articulatory speaking rate. Why are stuttered utterances longer than perceptibly fluent utterances? One reasonably straightforward answer to this fundamental question is that utterance length is a macrovariable that encompasses other speech production variables. Thus, in the present study, the length differences between stuttered and perceptibly fluent utterances may, at least in part, be attributed to grammatical complexity. Although present findings do not lend support to the idea that the encoding of grammatically complex utterances is necessary for an utterance to be stuttered, they do, however, allow for the possibility that the encoding of grammatically complex utterances may be sufficient for an utterance to be stuttered. How, then, might grammatical complexity be related to stuttered speech? Experimental evidence from speech error research (e.g., Garrett, 1980) and pause research (e.g., Boomer, 1965; Rochester and Gill, 1973) suggests that speech production plans are organized into phrasal and/or clausal units. One possible answer to this question, then, is that the encoding of grammatically complex clauses (such as those measured by the DSS) requires more cognitive resources than the encoding of grammatically simple clauses. That is, encoding grammatically complex utterances may place greater demands upon a speaker's capacity for grammatical encoding, and as the speaker's capacity for grammatical encoding becomes taxed, disfluent speech becomes more likely. Indeed, it has been demonstrated in several studies (e.g., Masterson and Kamhi, 1992; Nelson and Bauer, 1991; Panagos, Quine and Klich, 1979; Prelock and Panagos, 1989) that as complexity in one linguistic domain increases, performance accuracy within that domain tends to decrease as does complexity and performance accuracy in other linguistic domains. In this view, then, fluency breakdowns in persons who stutter might be symptomatic of such tradeoffs within and between linguistic domains. A second possibility concerns the time course of encoding grammatically complex clauses. That is, as Perkins, Kent, and Curlee (1991) suggested, longer formulation time at the grammatical level could, in turn, disrupt the integration of linguistic with paralinguistic information. In this view, then, fluency breakdowns in persons who stutter might be symptomatic of attempts to initiate production of an utterance prior to fully being able to formulate it. Other variables that are encompassed within utterance length and that have been proposed to be related to stuttered speech include aspects of phonologi-

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cal encoding (e.g., Kolk, 1991; Kolk et al., 1991; Postma, 1991; Postma, Kolk, and Povel, 1990a) and prosodic encoding (e.g., Klouda and Cooper, 1987, 1988; Prins, Hubbard, and Krause, 1991; Wingate, 1984). Differences between stuttered and perceptibly fluent utterances in terms of these latter variables were not, of course, examined in this study, but may warrant further research. On the surface, present data fail to support the common notion that fast speaking rates are characteristic of stuttered utterances, and suggest that strict adherence to slow articulatory speaking rates may not be crucial for fluent production of specific utterances. It is important to remember, however, that other rate-related variables at the discourse level (e.g., response time latency; Kelly and Conture, 1991) may be significantly related to the speech (dis)fluency of children who stutter. As suggested above, it may be the case that relatively long response time latencies permit a speaker to more fully and accurately formulate grammatical, phonological, and prosodic aspects of an utterance. Once these aspects of an utterance have been fully and accurately formulated, the rate at which the formulation is executed (i.e., articulatory speaking rate) may be of less relative importance to speech fluency. Such speculation warrants further empirical investigation. In the present study, 80% of these 15 subjects' stutterings occurred on the first word of an utterance. Accordingly, most articulatory speaking rate measures for stuttered utterances in the present study ended up assessing articulatory speaking rate after the occurrence of stuttered speech. Thus, present data also fail to support the notion that children who stutter may increase articulatory speaking rate in an attempt to compensate for time spent stuttering. It is quite possible, however, that a more precise measure of speaking rate relative to stuttering, such as one that considers speaking rate changes at a "local" or speech sound segment level (e.g., Viswanath, 1989), would have provided a more meaningful assessment of this issue.

Caveats

One limitation in the present study was the relatively small number of stuttered and perceptibly fluent utterances sampled per subject. This limitation became most apparent when the 25 stuttered and 25 perceptibly fluent utterances produced by each subject were then divided into four categories using the median split procedure. For several of the statistical tests in this study, especially those based upon the mean split procedure, mean differences between stuttered and fluent utterances, approached, but did not reach statistical significance. It is possible that with more subjects and a greater number of samples per subject (i.e., increased statistical power) these nearly significant differences related to length, grammatical complexity, and articulatory speaking rate would have reached statistical significance. Thus, one

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must be cautious in the interpretation of these results with regard to the differences between stuttered and perceptibly fluent utterances. Second, present findings regarding the relationship between grammatical complexity and speech fluency are valid only to the extent that DSS measures linguistic complexity. Although the DSS procedure has been found to reliably differentiate between language learning-disabled children and normally-achieving children (see Hughes, Fey, and Long, 1992 for a review), the DSS has several limitations. For instance, the DSS protocol requires that children use the "adult standard" for each grammatical structure assessed in order to receive complexity points, thus no partial credit can be given for emerging linguistic structures. Also, the DSS protocol specifies that only utterances containing both a subject and verb can be analyzed, thus data from phrase-level utterances cannot be considered. In the present study, an average of 65% (range 50% to 81%) of subjects' utterances met the subject-verb requirement for DSS scoring. It is possible that supplemental use of a procedure that addresses limitations such as those described above (e.g., Scarborough's (1991) Index of Productive Syntax) may have resulted in somewhat different conclusions about the grammatical complexity of stuttered and perceptibly fluent utterances in the present study. In the end, however, we may find that attempts to quantify multifaceted constructs such as grammatical complexity will always be somewhat imprecise as well as problematic. Indeed, a recent study by Kadi-Hanifi and Howell (1992) found that preschool-age subjects who stuttered were significantly more disfluent during grammatically simple utterances rather than during grammatically complex utterances, a finding which runs counter to the demands-capacities notion discussed earlier. SUMMARY AND CONCLUSIONS These data provide support for the idea that, for young children who stutter, stuttered conversational utterances are longer and more grammatically complex than perceptibly fluent conversational utterances. Utterance length, however, is in all likelihood a macrovariable that encompasses grammatical complexity, as well as other aspects of speech-language formulation and production such as phonologic and prosodic encoding. These data did not provide support for the notion that articulatory speaking rate differs between stuttered and perceptibly fluent utterances, nor did they provide support for the idea that articulatory speaking rate interacts with either utterance length or utterance grammatical complexity to influence whether or not an utterance is classified as being stuttered. Results support current intervention approaches whereby the length and grammatical complexity of young children's utterances are modulated to help (re)establish fluent speech production. Results do not appear to support the clinical practice of systematically

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reducing young children's articulatory speaking rates to (re)establish fluent speech production, although therapeutic interventions that alter other temporal aspects of speech production, such as "slow, gentle onsets" and longer response time latencies (i.e., increased turn-switching pauses), may still be of significant benefit and clearly warrant further empirical investigation.

This research was supported in part by OSEP H023C80008) and NIH (DC00523) grants to Syracuse University. The authors extend their thanks to Raymond Colton, Linda Milosky, Nan Bemstein Ratner, Meryl Wall, and one anonymous reviewer for their insightful reviews of earlier drafts of this article, to John Gleason and Randall Robey for comments and suggestions regarding statistical analysis of the data, and to J. Scott Yaruss for his assistance with interjudge reliability measures and data analysis. Special thanks are extended to the children and mothers who participated in this study. Portions of this paper were presented at the Annual Convention of the American Speech-Language-Hearing Association (November, 1992), San Antonio, TX.

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