Epidemiology of Stuttering inthe Community Across theEntire Life Span

Ashley CraigKaren Hancock

Yvonne TranMagali CraigKaren Peters

Department of Health SciencesUniversity of Technology

Sydney, Australia

A randomized and stratified investigation v/as conducted into the epidemiology ofstuttering in the community across the entire life span. Persons from households inthe state of New South Wales, Australia, were asked to participate in a telephoneinterviev/. Consenting persons were given a brief introduction to the research, anddetails were requested concerning the number and age of the persons living inthe household at the time of the interview. Interviewees were then given adescription of stuttering. Based on this description, they were asked if any personliving in their household stuttered (prevalence). If they answered "yes," a numberof corroborative questions were asked, and permission was requested to tapeover the telephone the speech of the person who stutters. Confirmation ofstuttering was based on (a) a positive detection of stuttering from the tape and (b)an affirmative answer to at least one of the corroborative questions supporting thediagnosis- Results showed that the prevalence of stuttering over the wholepopulation was 0,72%, with higher prevalence rates in younger children (1.4-1.44) and lowest rates in adolescence (0.53). Male-to-female ratios ranged from2,3:1 in younger children to 4:1 in adolescence, with a ratio of 2,3:1 across allages. The household member being interviewed was also asked whether anyonein the household had ever stuttered. If the answer was "yes," the same corrobora-tive questions were asked. These data, along with the prevalence data, providedan estimate of the incidence or risk of stuttering, which was found to range from2,1% in adults (21-50 years) to 2.8% in younger children (2-5 years) and 3.4%in older children (6-10 years). Implications of these results are discussed,

KEY WORDS: stuttering, incidence, prevalence, fluency disorders

Journal af Speech, Language, and Hearing Research

I t is important that clinicians, researchers, and health administra-tors know the prevalence and incidence (risk) of a disorder in thecommunity in order to allocate sufficient resources for managing proh-

lems associated with that disorder. Stuttering is a communication dis-order involving involuntary disfluency. It is ordinarily diagnosed early—around age 2 in the majority of cases—and it can become a chroniccondition for up to 20% of those who stutter in their childhood (Andrewset al., 1983; Bloodstein, 1995). Therefore, resources must he allocated tomanage stuttering in young children, adolescents, and adults. However,the extent of the population who stutter over the total life span is notclear, as the prevalence of stuttering in the community has only heenestimated hased on studies of children {Bloodstein, 1995).

In this paper, prevalence is defined as the numher of confirmed casesof stuttering in a sample at the time the sample is interviewed. This isknown as point prevalence (Slome, Brogan, Eyres, & Lednar, 1986, p. 34).

Vol 45 - 1097-1105 • December 20021092-4388/02/4506-1097

©American Speech-Language-Hearing Association ] 0 9 7

Incidence is usually defined as the occurrence of newcases durhig a specified period of time (Slonie et al., 1986.p. 98). Although the I'esearch reported in this paper isnot prospective in design and is therefore not primarilydesigned loassess the risk of stuttering (Moscicki, 19841.it may be possible to use the data to compute an esti-mate of the risk, 'fhis estimat(; was determined fromthe number of confirmed stuttering cases at the time ofinterview (prevalence) and the number of persons whostuttered in ihe past hut who did not stutter at the timeoi'the interview. Incidence is, therefore, defined as theT-isk of the person in the sample population ever stutter-ing over the period of the person's life span; it is basedon confij-med cases of current stuttering as weU as casesof past stuttering.

This paper does not attempt to review the largenumber of prevalence studies that have been conductedover the past century. For this, the reader is referred toBloodstein (1995, pp. 105-107), who has provided a suc-cincl review. The aim of this paper is to analyze a smallnumber of more recent studies that have contributed tothe current understanding of the epidemiology of stut-tering. The review concentrates on prevalence rates inchildren, adolescents, and children/adolescents with dis-ability in addition to stuttering, in that order. The sexratio and risk of stuttering are also addressed.

Although a number of studies determined childhoodstuttering prevalence, there are few, if any, randomized,stratified studies that investigated the extent of stut-tering in the total population (Andrews et al., 1983;Bloodstein, 1995). ('urrently, estimates of prevalenceare based on cross-sectional or survey research onschoolchildren. For example, Andrews and Harris (1964)surveyed 7,358 schoolchildren from 9 to U years old inNewcastle upon 1>'ne, England, They relied on school-teachers to identify children who stuttered and thenassessed each child selected by the teacher. Although86 children were identified by teachers, only 80 chil-dren were subsequently confirmed as children who stut-ter. Bloodstein (1995, p. 107) reports a prevalence of1.2'> for this study (based on 86 children), but the cor-rect prevalence rate is almost 1,1'7( based on 80 chil-dren. Leavitl (1974) conducted two surveys amongPuerto Rican schoolchildren. The first was in 12 schoolsin San Juan, Puerto Rico iN = 10,449, comprising 5,476boys and 4,973 girls). The second involved Puerto Kicanchildren from 19 schools in New York iN = 10,455, with5,270 boys and 5,185 girls), Leavitt also relied on school-teachers to identify children who stuttered and thenconducted assessments to confirm the diagnosis. Shefound an overall prevalence rate of 1,5';; m the PuertoRicau children living in San Juan (2.17% in boys and0,769; in girls) and an overall rate of 0.84% in the PuertoRican children living in New York (1.44% in boys and

0.23%. in girls), Aron : 1962), using teacher referral toselect children who stutter, sur\'eyed 6,581 schoolchil-dren (3,105 males and 3.476 females IVom 6 to 21 y(>arsold) in 13 schools m Johannesburg and found an over-all prevalence rate of "1.26'r (83 who stuttered. 62males and 21 females). Gillespie and Cooper 1,1973)surveyed junior high and high school students in Ala-bama, They interviewed 5.054 students (grades 7-12by convenience sampling) and found a prevalence rateof 2.1%, Boyle, Uccoufle, and Yeargin-Allsopp (1994)conducted a large survey o f l7 , l ]0 children, from thevery young to age 17, (Children were invited to partici-pate from a register estahlished by the National HealthInterview Survey (NHIS), The NIIIS involves an on-going survey of households within the United Statesand uses a sampling procedure that is believed to berepresentative of the civilian non-institutionalizedpopulation (Boyle e t a l , 1994). The study was designedto assess a number of disabilities, such as developmen-tal disabihty, deafness, learning disability, and stut-tering. The researchers found a stuttering prevalenceof 1.89% (A'=297), In contrast, Brady and Hall (1976)conducted a survey on a very large sample nV =187,420) of schoolchildren in Illinois and found only a0.35" ^ prevalence rate.

Of the above seven studies, the nu:an pr'evalencerate in children is 1,29%. However, Bloodstein (1995)listed 17 U.S, studies that report a mean prevalence rateof 0.97% for school-age children and 21 non-U.S, stud-ies that report a mean prevalence rate of 1,28'; , Andrewset al. (1983) suggested the difference between the tworates may he due to the higher proportion of U,S, chil-dren who remain in school after puberty, (^inversely,the difference may also be due to error arising from thesampling and assessment methods u.sed by many ofthese studies. When the sample is not randomized andstratified, it is not a reliable representation of thehroader population.

Although adolescent prevalence rales have; rarelybeen studied, Ardila et al, (1994) found a 2% prevalencerate in 1,879 Spanish-speaking university students.Porfert and Rosenfield 11978) found a 2A'.r prevalencerate in 2,107 students at a U,S. university. However,both studies employed non-randomized sampling,thereby increasing the likelihood of obtain ing unrepre-sentative samples, and both studies failed to confirmthe stuttering cases.

Prevalence rates have varied widely in studies con-ducted on populations with disabilities, Stansfield (19901conducted a non-randomized survey of adult psychiat-ric wards and relied on medical staff to send back infor-mation on patients who had speech problems. Stansfieldthen assessed tho speech of a percentage of those believedto have speech problems and found a 6.3';v prevalence

1098 Journal of Speech, Lc^guage, and Hearing Reheard, • Vol.45 .1097-1105 • December 2002

rate of stuttering in tbe adult psycbiatric population.Montgomery and Fitcb (1988) sent survey forms to 150schools for the hearing impairetl. Only children who badbeen diagnosed as having a stutter by a qualified speecbprofessional were counted in the survey. Just over halftbe schools replied. Only 12 cbildren out of a total of9,930 were reported to have a stutter, and all but onehad congenital hearing loss. Montgomery and Fitcbfound a prevalence rate of 0.12% in scbool-age childrenwith hearing disorders. Due to tbe non-randomized sam-pling metbods used, these two studies do not providereliable stuttering population rates for disabled persons.

Male-to-female ratios were investigated by Aron(1962), wbo found a 2.9:1 (maleifemale) child ratio;Andrews and Harris (1964) found a similar ratio.Gillespie and Cooper (1973) found a cbild male-to-fe-male ratio of 2.7:1, and Porfert and Rosenfield (1978)found a ratio of 2.4:1 in university students. The major-ity of studies have found child stuttering male-to-femaleratios of about 2.5:1 to 3:1 (Bloodstein, 1995).

Incidence studies are less common tban prevalencestudies. Andrews and Harris (1964) investigated inci-dence in a sample of 1,000 cbildren followed from birthup to age 15 in England (incidence was defined as thepercentage of the 1,000 cbildren who stuttered at anytime in their life). However, there was a 25.8% attri-tion rate over the first 5 years. The resulting sample of875 included 43 children who were thought to stutter,providing a 4.9% risk estimate. Ingham (1976) ques-tioned the reliability of the incidence data from thisstudy for several reasons, including uncertaintywhether 16 of the 43 children actually stuttered. If the16 are removed, the estimate of the lifetime risk of stut-tering in the Andrews and Harris (1964) study reducestoabout3.19f.,

Mansson (2000) studied the incidence of stutteringin Denmark. All children born on the island of Bornholmin 1990 and 1991 were assessed for stuttering when theyturned 3 years old. The total population of this islandwas 45,000. Although 1,042 children were born, only1,021 participated in the third year. Fifty-one were be-lieved to stutter, and only two additional cases were iso-lated in surveys conducted 2 years later (age 5) and 4years later (age 9). As a result, Mansson suggested theincidence for this group of children was 5.19%. This studyis to be commended for its prospective design and sam-pling rate. However, Mansson acknowledged that theisland's geographic isolation and homogeneous ethnicpopulation limit the generalizability of the findings.Mansson also failed to report reliability of the assess-ment of stuttering or a definition of stuttering. Basedon a number of studies, Bloodstein (1995) and Andrewset al. (1983) concluded that the risk of stuttering isaround 4 to 5%.

In summary, little is known about the prevalence orincidence of stuttering throughout the entire life span.It is important to conduct research designed to estimatereliably the number across all ages as well as the sexratio of those persons who stutter in the community. Thispaper presents a study that investigated the prevalenceand sex ratio of stuttering in an ethnically heterogeneoussample. The study also provided an estimate of the inci-dence of stuttering.

MethodParticipants and RandomSampling Procedure

The study consisted of a random and stratified selec-tion of households in New South Wales (NSW), Australia.The population of NSW consisted of approximately 6 mil-lion people during 1995 to 1996 when the data were col-lected. The population was composed of primarily city andsuburban dwellers (74%). Almost 77% of people in NSWwere born in Australia, though the population is ethni-cally diverse, with the most common groups in NSW inrank order being people of European/British, Asian,Middle Eastern, and Indian descent. Families living incity, suburban, and rural areas across NSW were randomlyselected, so that (a) all families had equal chances of be-ing selected and (b) the distribution of people in the samplefrom these three types of areas was proportional to theknown population spread in NSW,

Our hypothesized proportion of people who stutterin the community was 1% (0.01), and we planned toidentify up to 100 people who stutter (as this wouldprovide sufficient and stable numbers of people whostutter to determine the epidemiology of stuttering).This imphes we needed a sample size of 10,000 (100/0.01) people from whom to collect epidemiological dataregarding stuttering. As stuttering is now believed tobe genetic in origin (Ambrose, Cox, & Yairi, 1997), therandomness of the selection process could be negativelyinfluenced hy cluster sampling. Therefore, we aimed tocollect a larger sample than 10,000 in order to over-come any possible influence of cluster sampling. Tbefinal number of participating families was 4,689, con-sisting of 12,131 individuals.

Families were selected (using 1995/96 telephonedirectories) until enough households had been contactedwho agreed to participate in the survey. In accordancewith the population distribution of NSW (AustralianBureau of Statistics, 1998), three-quarters of the samplewere from city/suburban areas in NSW, whereas theremaining sample came from rural areas in NSW. TableI shows the regions sampled. If a survey is strategi-cally conducted according to known distributions of a

ig et ol.: Epidemiology of Stuttering t 0 9 9

Table 1 . Breakdown of regions in New South Wales that were

selected for the survey.

Region

Sydney regionCentral coastSouth coastWindsor areaCampbelltown areoPenrith areaSouthern tablelandsAlbury areaBathurst areoCooma areaSingleton areaNorthern riversNew England areaWestern plainsSouthwest plainsBroken Hill areaTotal

Males

3,62527711185

28929810396

17678

28818615810212835

6,035

Females

3,648

25911580

272

312

no112

16979

269

173186

119160

316,096

Total

7,274

536226165561

6102)3208

346157557

359344

221288

6612,131

Nofe. The number of persons interviewed in each region v/as based onthe populatian distribution across the state so that the numbers sompledin each areo of the study were propartional to the population spread(total N = 12,131).

population, then regional coverage rates ar(! higher andtherefore are helieved to be valid representations ofthe population (Cannell, 19851.

Households were contacted by telephone and inter-views were then conducted (either during this initial con-tact or at a convenient lbllow-up time). As over 95'/; ofAustralian households had a telephone m 1995/96, a highpenetration rate in the community was assured. There-fore, only a small chance existed of introducing popula-tion bias into the sample. The procedure used (Dillman,1978) involved the estimation ofthe total length of col-umns of listings for each ofthe telephone directories usedin NSW. A sampling distance Iin centimeters) was com-puted by dividing this estimated length by the proposedsample size from that region. For each telephone direc-tory, a different starting point was selectd usitig randomnumbers. This proposed sample .size was then used asthe sampling interval between each pair of selected tele-phone numbers in the telephone books. Using this proce-dure ensures that most of names in each directory havethe potential to be selected for the study (Dillman, 1978).Table 1 shows the breakdown for the regions sampled aswell as the final sample sizes surveyed from eacb region.Families in which no metnbei" could speak English wellenough to complete the interview were not, included inthe study. If tbis occurred, tbe interviewer noted this andproceeded to another number. Telephone numbers thatwere disconnecte(i or where there was no answer afterthree attempts were also noted and the same procedure

was followed (see results for response rate). The time andday of interviews was varied through the week to ensurea high penetration rate.

Stuttering Definition andInterviewing Procedure

Pive professionals were trained to conduct the ln-ten-'iews. An interview began with a brief scripted state-ment of the purpose of the survey and identified theuniversity represented. The scripted introduction in-cluded informed verbal consent—tbe interviewer askedthe person who answered the telephone for permissionto be inter\-iewed. If the person gave consent but saidthat another time was more convenient, a new time wasarranged for tbe interviewer to call. If a young childanswered the telephone, the interviewer asked to speakto a parent for the interview. All refusals were recordedas missing data as were "no answers." Tbe mterviewwas structured to include forced-choice questions so tbatresponses could be categorized. Tbe results from eacbtelephone interview were recorded on provided responseforms (see tbe questionnaire in the Appendix).

Stuttering was defined to all inter\'iewees in detailusing a standard definition (Craig et al., 1996). It wasdefmed as repetitions of syllables, part or whole wordsor phrases; prolongations of speech; or blocking ofsounds. Associated symptoms such as embarrassmentand anxiety were also discussed, [f requested, tbe inter-viewer gave a demonstration over tbe telepbone of arepetition and a block. Tbe interviewees (if not children)were asked whether they or a member of tbeir house-bold stuttered. If tbey were not sure, they were encour-aged to speak to other members ofthe family and analternative time was arranged to call to complete tbeintei-view. If tbe people answering the telephone believedthey or a member of their household stuttered, a num-ber of corroborative questions were then asked directlyoftbe person believed to stutter, or of parents if a youngchild stuttered. Questions included (,a) "Has tbe stut-tering persisted for tbe tast thnse months?", (b) "Hastbe stuttering caused fear and avoidance of situations?",(c) "Has tbe person consulted a speech professional?",and (di "Has tbe person had tberapy for stuttering?" Ii'someone in the family was believed to stutter, the inter-viewer asked for permission to speak to that metnberand to tape tbe person's speech over tbe telepbone forup to 5 minutes. Tf the person was a child, tbe inter-viewer asked tbe parents for permission to tape tbeircbiid's speech. Taping involved tbe interviewer engag-ing tbe person believed to stutter in conversation for atleast 5 minutes, during wbicb tbey taped the person'sspeech. During tbis interview, tbe interviewer also lis-tened for stuttering.

1 1 0 0 Journal of speech, Language, and Hearing Research ••h • Vol 45 • 1097-1105 • December 2002

As a person who stutters may be less likely to an-swer the telephone, it is probable that someone in thehouse who does not stutter will answer. If the personanswering believed he or she or someone else in thehousehold had stuttered at some previous time (that is,is not presently stuttering), the interviewer asked forpermission to speak to the person who formerly stut-tered and asked this person similar corroborative ques-tions. In most cases, the interviewer was able to speakto the person who was believed to have formerly stut-tered. If not, the interviewer requested the informationfrom a person in the household who could answer thecorroborative questions.

ReliabiUfy of the Inferviews

Reliability of the survey itself was also an impor-tant consideration, and a 1-week test-retest reliabilitymeasure conducted on 15 interviews showed 100% agree-ment on the prevalence interview. This involved recon-tacting families who had participated in the study. In-terviewers were trained in the interview protocols, withemphasis placed on establishing rapport with the re-spondent. Interview structure and rapport are impor-tant in ensuring the validity and reliability of telephonesampling compared to face-to-face and self-administeredmodes (Quine, 1985). Traditionally, personal interviewsare regarded as more valid and reliable than telephoneinterviews. However, several studies reported no signifi-cant differences between these modes of interview inoutcome and also in socio-demographic data obtained(Aneshensel, Frerichs, Clark, &. Yokopenic, 1982;Cannell, 1985; Paulsen, Crowe, Noyes, & Pfohl, 1988;Quine, 1985). Low response rate is a possible disadvan-tage when conducting telephone interviews, but it is adisadvantage that besets all modes of interview. Stud-ies conducted in the United States report lower responserates of only 3-5% for telephone interviewing comparedwith face-to-face interviewing (Cannell, 1985). Telephonesamples give higher response rates than mail surveysand share many of the advantages of the face-to-faceinterview over the self-completed questionnaire. In thisstudy, no one who stuttered refused to be interviewed.

Validation and Reliability ofStuttering Assessment

An accurate determination of prevalence and riskdepends on a valid and robust method of detecting stut-tering. The five interviewers were trained in the inter-view protocol and in the detection of stuttering. All fam-ily or household members who were believed to stutterwere consequently interviewed for up to 5 minutes oruntil at least 500 syllables were taped. During the in-terview, and based on the above defmition of stuttering,

the interviewer decided whether the person stuttered.The interviewer taped the speech using digital technol-ogy (Sony Pro IITCD DIO, Sony condenser microphone,ECM-MS5) to ensure high-quality recording. The inter-viewer then passed the tape to a co-researcher "rater"who had over 15 years of experience in treating and di-agnosing stuttering. The rater qualitatively and quan-titatively evaluated the tape of the participant's speech.If both the interviewer and the rater agreed that theperson stuttered, they determined the frequency of stut-tering (percent syllables stuttered or 9? SS) and speechrate (syllables per minute or SPM). This was the pri-mary measure of stuttering. To ensure reliability, anindependent rater (that is, neither the interviewer northe original rater) assessed a proportion of the ratedtapes for %SS and SPM. The interrater reliability of therater was demonstrated. The interrater reliability, us-ing Pearson correlation coefficients with the second ex-perienced rater, was 0.93 for %SS and 0.91 for SPM on15 randomly selected tapes (that is, 17% of the totalnumber of tapes). If stuttering was diagnosed from thetape, the corroborative evidence mentioned above wasthen used to confirm the diagnosis of stuttering. Therater and independent rater demonstrated over 96%agreement on the diagnosis of stuttering from the tape.When there was disagreement or no corroborative evi-dence, the case was not considered confirmed as a per-son who stuttered. Only confirmed stuttering cases wereused in the analysis to determine prevalence and to es-timate incidence. Obviously, taping speech on the tele-phone will not assess secondary aspects of stutteringsuch as facial grimace, and so on. However, secondarysymptoms are extremely variable and are not generallyconsidered necessary to measure frequency of stutter-ing (Bloodstein, 1995).

ResultsResponse rate for telephone calls was considered

satisfactory, with 63% of the telephone numbers initiallyselected resulting in completed interviews (for a minor-ity of numbers interviews were completed after 2-3calls). This meant that for 37% of initially selected tele-phone numbers, a second or third alternate telephonenumber had to be randomly selected. Of the 37% of callswhere interviews did not occur, 19% of respondents re-fused, giving no reason, 0.15% were too busy or unwell,and 6.9% did not speak English well enough to com-plete an interview. The remaining 10.95% could not becontacted due to disconnected lines or unanswered calls.After three attempts without contact, a new number wasselected.

The mean %SS for the sample of those who stut-tered in = 87) was 5.04 %SS (range of 0.5 to 24.7, SD =

Craig etol.: Epidemiology of Stuttering 1 101

3.8), and tbe mean SPM was 169 (range of 88 to 218..SD = 26). All inter\newees whose stuttering was con-firmed were asked if tbey had ever sougbt treatment(e.g,, speech pathology, medical, psycbological, bypno-tlierapy) for their stuttering, and around 30% repliedtbat they bad at some time received treatment (justover ^0% bad never received treatment and about 20%were unsure). Frequency of stuttering was sbown tocorrelate significantly and positively witb seeking treat-ment (point biserial r = 0.23, /; < .05). Tbis suggeststhat those with more severe stuttering had sought treat-ment. Table 2 also sbows age and sex breakdowns intbe sample. The sample of 4,689 families consisted of12,131 persons (6,023 males, mean age 35,5 years, SD= 21.6, age range from less than 1 year to 95 years;6,108 females, mean age 37.3, SD ^ 22,1; age rangefrom less tban 1 year to 99 years). There was an aver-age of 2,9 members in eacb of the 4,689 families inter-viewed. There was a total of 1,638 participants age 10or younger 113.5'/ )—though only children who were atleast 2 years old were included, as stuttering usuallybegins around age 2—resulting in a sample of 1,622.There was a total of 1,881 participants age 11 to 20(15.5'7'ri, and the majority of participants [n ^ 8,612)were age 21 or over (71% ).

Table 2 also shows prevalence rates as well as male-to-female stuttering ratios for all age categories. Preva-lence was highest in the 2-5 (1,4^ ) and 6-10 (1.44';f)age groups and lowest in the U~20 (0.53%) and 51 andolder (0.37%) age groups. The overall prevalence ratefor tbe entire sample was 0.72' (95% confidence inter-vals (Cls) of 0,57 to 0.87). Bloodstein (1995) presenteddata on the prevalence in scbool-age cbildren (approxi-mately 5-18 years) of around 1%. In this study, theprevalence of stuttering in this same age group (5-18years inclusive; u = 2553 with 23 persons who stuttered)was found to be 0.9 ^ (95%^ Cl of 0.54 to 1.26%). Table 2also sbows sex ratios by age group. Over the total sample,the male-to-female sex ratio was 2.3:1. It was highest inthe 11-20 age group (4:1 male:female) and lowest in the51 and older age group (1.4:1),

Prior cases of stuttering (those believed to stutterin the past) are also sbown in Table 2 , This allows es-timation of the incidence of stuttering. Risk over theentire sample was 2.2'/^. witb, as would be expected,the highest risk in young children (2.8%' in 2- to 5-yoar-olds) and older children (3.4';{ in 6- to 10-year-olds),Lowest risk was in those over 50 years old (1,8'/r), Tberisk in 5- to 18-year-olds in = 2553 with 71 current-stuttering and prior-stuttering cases) was determinedto be 2.8% (95% Cl of 2,16 to 3.44). Data on recoveryrates can also be derived from tbis researcb. Table 2sbows that there were 87 people identified as currentlystuttering and 176 identified as stuttering in the past.Tberefore, these data provide a 679; recovery rate overthe entire life span (176/263).

Discussion and ConclusionTbis paper presents epidemiological data on the

prevalence, sex ratio, and estimated risk of stuttering.The study was conducted within an etbnically hetero-geneous population in Australia, strengtbening its ap-plicability to other populations tbat are also ethnicallyheterogeneous. Furthermore, it provides the first at-tempt to estimate tbe prevalence of stuttering acrosstbe entire life span using randomly selected householdstbat are stratified for region (city, suburban, and rural),

Tbe prevalence of stuttering over tbe entire lile span(from age 2 on) was found to be 0,72'/f witb at least a507r liigber prevalence rate of stuttering in males (2.3:1male-to-female ratio), A 0,72'i'f prevalence rate is rea-sonable given tbat it is accepted that many childrennaturally recover from stuttering (Bloodstein, 1995). Ahigber prevalence rate of around 1.4'/r was found in cbil-dren (2 to 10 years), with male children again having abigber prevalence of stuttering (2,3 to 3.3:1). In adoles-cence (11 to 20 years), tbe prevalence rate fell substan-tially to 0.53%, witb boys mucb more likely to stutter(4:1 ratio). However, prevalence increased in adulthood(21-50 years) to 0.787r (2.2:1 ratio), falling once again

Table 2. Breakdown of age by stuttering cases (SC), prior stuttering cases (PSC), prevalence, male-to-female stuttering ratios, and on estimate

of the incidence or risk of stuttering.

Age (yrs)

2-5

6-1011-2021-505 U

All ages

Note. There are

SC

10

1310

421287

16 missing

PSC

10

18

327145

176

cases in the 2-5

Total N

720902

1,8815,4053,207

12,131

# males

389465

1006260715566023

age breakdown as children

Prevolence (95% Cl}

1,4%

1,44%

0,53%

0.78%

0,37%

0,72%

(0,54-2.26)(0.66-2,22)[0,20-9.86)(0,55-1.01)(0,16-0,58)(0,57-0.87)

less than 2 years old were not

M:F ratios

2,3;13,3:1

4:12,2:11.4:12.3:1

Risk

2,8%

3.4%

2,2%

2.1%

1.8%

2.2%

included in the analysis.

(95% Cl)

11.6-4 0)

(2.2-4,6)(1,5-2.9)[1.7-2.5)(1.3-2,3)(1.3-2,3)

1102 JotJniol of Speech, Language,, and Hearing Research • Vol 46 • 1097-1105 • December 2002

in late middle to older age (0.37% for 51+ years), withmales again stuttering more frequently than females(1.4:1 ratio). It is interesting to note the 959f confidenceintervals for all the prevalence data in Table 2. Theprevalence rates found by the majority of studies re-viewed in this paper fall within these 95% confidenceintervals. It is also interesting to note that the 95% con-fidence intervals for the prevalence of school-age chil-dren found in this study (5-18 years; 0.9%. prevalence,Cl range from 0.54% to 1.26%) covers the estimatesquoted by Bloodstein (1995), wbo suggested that theprevalence in tbe United States was 0.97% and outsideof tbe United States was 1.28%.

The results of tbis study bighligbt two possibilities,that (a) tbe accepted prevalence of around 1% in youngercbildren is too low and (b) the accepted prevalence ofaround 1% in adolescence is too high. Tbe prevalencerates in Tahle 2 suggest tbat more realistic figures mayhe 1.4%j in younger children and only 0.5% in adoles-cents. Tbe 1.4% prevalence rate in children found in thecurrent study is substantially higher tban Bloodstein's(1995) estimate and is closer to tbe mean prevalencerate of 1.3% from the seven studies critiqued in the firstsection of this paper. The results of the current studystrongly suggest a higher prevalence rate in youngerchildren be accepted as the status quo. Significant clini-cal issues arise that will affect tbe management of stut-tering in younger children if the prevalence of the disor-der is underestimated. For example, assuming that ] 4%of a population of 200 million people falls in the age rangeof 2 to 10 years, a 1%. prevalence rate of the number ofcbildren who stutter at any one time is 300,000. How-ever, if the prevalence is at least 1.4%, then the numberof cbildren stuttering is underestimated by 120,000(420,000 rather tban 300,000 children may be expectedto stutter). If this underestimation of prevalence weretbe case, then tbe clinical load for clinicians treatingcbildren would be bigh, witb long wait times. Servicesfor adults who stutter would be severely reduced as aresult. Tbis clinical scenario is a reality in Australia.

Tbe lower prevalence rate in adolescents should notbe unexpected given that tbe trend in tbe treatment ofchildren who stutter is to treat the stutter early ratherthan wait hoping tbat spontaneous remission will occur(Lincoln & Onslow, 1997). In addition, a significant pro-portion of children's stuttering is thought to spontane-ously remit hefore adolescence (Yairi & Ambrose, 1992).Recovery from stuttering bas been estimated to be atleast 70% from longitudinal researcb {Yairi & Ambrose,1992) and is now believed to be botb more frequent infemales and genetically transmitted (Ambrose, Cox, &Yairi, 1997). Tbis study sbows an estimated recoveryrate over tbe entire life span of almost 70%. Tbe increasein prevalence tbrougb early to middle adultbood may

also be due to relapse (Craig et al., 1996), or to otherfactors not yet understood. Perhaps it also reflects agreater willingness to discuss ones' stuttering. The de-crease in prevalence in older ages cannot yet be ex-plained. As found elsewhere, more males than femalesstuttered, but the sex ratio also changed over the lifespan (a 3 or 4 to 1 male-to-female ratio in older cbil-dren, with an expected smaller ratio in younger chil-dren and perhaps unexpectedly, down to 2.2:1 in earlyto middle adulthood, and further down to 1.4:1 in olderadults).

Risk of stuttering is best generated from prospec-tive designs in wbich new cases of stuttering are deter-mined over a designated period of time in a populationtbat is initially nonstuttering. This study provides onlyan estimated risk of stuttering, as risk was partly basedon the memories of those interviewed (those who werebelieved to stutter in tbe past). As a result, tbis studycould only provide a lower bound on the risk of stutter-ing due to the above design limitations and tbe assump-tion that people are unlikely to stutter after adolescence.The estimated risk in children ranged from 2.8%. to 3.4%in the present study. This is lower tban expected if oneaccepts tbe findings of Andrews and Harris (1964) orMansson (2000) tbat tbe risk is closer to 5% in cbildren.Inspection of Tahle 2 shows tbat the 95% confidence in-tervals came close (upper limit of 4%.) to the figures foundby tbe above authors. However, it may he, as arguedabove, that risk in tbe Andrews and Harris study is closerto 3%, a figure tbat concurs witb tbe findings in thepresent study. The Mansson finding of 5% may be over-estimated due to design limitations or may truly reflecta bigh risk in an isolated population. Further researcbis needed to determine tbe risk of stuttering, using aprospective study conducted on a randomly selectedheterogeneous population in which stuttering is care-fully diagnosed and assessed. There are no existing dataon risk throughout tbe life span, so tbe findings of thecurrent study await replication. However, as expected,the adult risk of stuttering was around 2%, almost 1%;less than the cbildbood risk. The risk was higher in olderchildren (3.4%) than in young children (2.8%), perhapsdue to new cases emerging in this older age group. Natu-ral recovery seems to have influenced risk in adolescents(2.2%). In a population of 140 million adults (if adultsrepresent 70% of 200 million people), a potential 2.8million people have a risk of stuttering. Furthermore,tbere may be from 600,000 up to 1 million people stut-tering at any one time in such a population. Given thepotential for stuttering to binder tbe psychosocial andemployment prospects for persons wbo stutter tbrougbtheir lifetime (Craig, 1990; Craig & Calver, 1991), thesefigures argue for the allocation of substantial resourcesto treatment facilities for adults wbo stutter.

Craig e^al.: Epidemiology of Skittering 1 103

Acknowledgments

This research was funded hy The University of Technol-ogy, Sydney, and a Commonwealth Department of HealthGrant (NHMRC), Thanks also to the following fundingbodies who contributed financially to tbe research: the BigBrother Movement, the Australian Rotary Health ResearchFund, the Sunshine Foundation, and the Inger Rice Founda-tion,

References

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Received May 15.2002Accepted July 15, 2002DOl: 10.1044/1092-4388(2002/088)Contact author: Ashley Craig. PhD, Department of Health

Sciences, University of Technology', Sydney, Broadway,New South Wales, Australia 2007,K-maii: a,craig#uts,edu,au

1 1 0 4 Jcrno! of Speech, Long^age, and Hearing Research • Vol 45 • 1097-1105 • December 2002

Appendix

Prevalence Questionnaire

Date / /

Stuttering Telephone Survey

Demographics

(a) May ! ask how many females live in your household atpresent?

(b) What are their ages?

(c) How many males live in your household at present?

(d) What are fheir ages?

The lettered questions could be answered Yes, No, or Not sure.

1. Do you stutter?

If answer to Q l is no, go to Q2.

(a) Has the stuttering persisted for the past 3 months?

(b) Has the stuttering caused fear and avoidance ofsituations?

(c) Have you consulted a health professional? (speechtherapist, psychologist, psychiatrist, GP, etc)

|d) Have you had therapy for stuttering?

Do you know what types of therapy? .

(e) Do you know whether anyone in your close familyever stuttered? (parents, grandparents, sisters,brothers, children)

Who are they?

If the person stutters, ask permission to tape for up to 5min.

2. Have you ever stuttered?

If answer to Q2 is no, go to Q3.

(a) Did the stuttering persist for at least 3 months?

(b) Did the stuttering cause fear and avoidance ofsituations?

(c) Did you consult a health professional? (speechtherapist, psychologist, GP, psychiatrist, etc.)

(dl Did you have therapy for stuttering?

Do you know what types of therapy?

(e) Do you know whether anyone in your close familyever stuttered? (as above)

Who are they?

3. Does anyone in your household stutter?

If answer to Q3 is no, go to Q4.

(a) Who are the persons in your household who stutter?(label F1,M2, etc.)

(b) Has the stuttering persisted for the past 3 months?

(c) Has the stuttering caused fear and avoidance ofsituations?

(d) Has the person(s) consulted a speech professional?

(e) Has the person(s) had therapy for stuttering?

(f) Does the person(s) have close family who have everstuttered?

If a person in the household stutters, ask permission totape them for up to 5 min.

4. Has anyone in your present household ever stuttered?

(a) Who are the persons in your household who havestuttered in the past? (label F l , M2, etc.)

(b) Did the stuttering persist for at least 3 months?

(c) Did the stuttering cause fear and avoidance ofsituations?

(d) Did the person(s) consult a health professional?(speech therapist, psychologist, GP, psychiatrist,etc.)

(e) Did the person(s) have therapy for stuttering?

(f) Do you know whether anyone in your close familyever stuttered?

Who are they?

Craig etal.: Epidemiology of Stuttering 1 105