of Epidemiology Study of SLI
with Specific Language Impairment (SLI) have unexpected and unexplained difficulties
learning and using spoken language. Although these difficulties are most apparent
during the preschool and early school years, evidence now exists that these
problems are usually present well into adulthood and are probably present
throughout the person's life. Associated with difficulties in spoken language,
first evident during early childhood, are impairments in reading and writing.
As a result, those individuals presenting this condition are at high risk
for academic difficulties. Despite life long academic, social and occupational
consequences on those affected, there is little information about the prevalence
of this condition or factors that place persons at risk for SLI. The few studies
that do exist suffer from small sample sizes, poor sampling design and inadequate
diagnostic procedures; furthermore none of these studies have been performed
in the United States. The NIDCD, through a contract with The University of
Iowa, has conducted an epidemiologic study of SLI in 5-year-old children based
upon a sample of more than 7,256 children in the states of Iowa and Illinois.
This study was designed to address the following issues:
Review existing literature on the epidemiology of SLI and related disorders.
Establish a definition of specific language disorder that is consistent with
current research and clinical practice. In accordance with this definition,
develop an explicit criterion for the diagnosis of specific language disorder.
Estimate, based upon this definition, the preva1ence of specific language
impairment in five-year-old children each of three community environments
in the United States: urban, suburban, and rural. Furthermore, estimate this
prevalence rate in both males and females within these strata.
Characterize for each stratum the percentage of children with SLI who had
primarily receptive primarily expressive, and receptive-expressive SLI. Also,
determine the proportion of children with SLI within these strata who had
concomitant phonological disorder.
Evaluate potential risk factors for specific language disorder.
Determine the history of clinical intervention for those children within each
regions of the state of Iowa and one from Illinois, each centering upon a
metropolitan area constituting an SMA (Standard Metropolitan Area) were used
to obtain subjects from each of these strata.
these metropolitan areas were Waterloo/Cedar Falls/Cedar Rapids (IA), the
Iowa-Illinois Quad Cities (Davenport/Bettendorf, IA, Moline, Rock Island,
IL, and Des Moines, IA.). Within each of the strata and sampling region, all
children who were age eligible for kindergarten during the study year (between
5 and 6 years old on September 15) were equally likely to be sampled. A stratified-cluster
sampling method was used where the cluster unit was the attendance zone for
a public school within one of the three community strata. The schools within
each sampling region and strata were randomly selected and all kindergarten
children attending the school were included in the study kindergarten children
attending the school were included in the study. Using this sampling scheme
over 7,000 children were sampled for participation in this study.
two-stage assessment procedure was used to obtain the data needed to address
the questions concerned with prevalence and risk for SLI. In the first stage,
a brief language screening test was administered to all kindergarten children
attending the schools sampled. The second stage of the assessment consisted
of a diagnostic examination for SLI, administered to each child who failed.
The screening test, as well as any child who had passed the screen and was
randomly sampled from the same strata and community as the screening failure.
The diagnostic examination consisted of a set of language measures, phonological
measures, two subtests of the Wechsler Preschool and Primary Scale of Intelligence-R
(Wechsler, 1989), and a pure tone hearing test. Additional information about
the child's language background, history of speech and language intervention,
and performance on pre-reading tasks was also obtained. In order to obtain
information bearing on risk factors placing children at risk for SLI, the
parents of each child who was found to present SLI, as well as 4 normal-control
subjects per SLI child, were then contacted by Ames Statistical Laboratory.
A 45-minute telephone questionnaire was administered to obtain background
information pertaining to potential risk factors for SLI.
Summary & Introduction
The results in this summary will be collapsed over the residential stratification
Over the duration of the study, 7,844 children were eligible to participate,
626 of these children were not screened for one of three reasons: (1) they
were non-English speaking or were from homes in which English was a second
language, (2) they presented with unequivocal exclusionary conditions such
as blindness or deafness, or (3) their parents did not wish them to participate.
Of the 7,218 children who were screened, 1,933 failed the screening and 5,285
passed the screening. Of the 1,933 children who failed the screening, 1,835
were monolingual English speakers. Of the 5,285 children who passed the screening,
5,159 were monolingual English speakers. Thus, among monolingual English speakers
73.9% passed and 26.1% failed the screen.
the 3,877 children selected for the diagnostic (1,933 screening failures and
1,944 screening passes), 2,084 were given permission by their parent's to
participate in the study. Of these children, 75 were reported by their parents
to speak a second language. The rest of the 2,009 children constituted the
final monolingual English speaker sample. The screening status for 901 of
these monolingual English speakers was fail and for the rest (1,108) it was
pass. However, 49 (39 of them had failed screening and the rest, 10, had passed)
of these 2,009 monolingual English speakers were reported to have exclusionary
conditions such as autism, head injury, mental retardation, etc. Although
these children were not administered the diagnostic protocol, they were included
in the denominator for calculating the prevalence of SLI. The diagnostic results
of the remaining 1,960 children who were administered the diagnostic protocol
showed that 1,479 had normal language status, 31 failed the hearing criteria,
215 presented language impairment but also failed the non-verbal cognitive
criterion and 216 qualified as SLI by presenting a language impairment in
the context of normal non-verbal IQ, normal hearing and no other exclusionary
data above provided the raw frequencies necessary to compute the prevalence
of SLI. Due to the nature of the study design in which all screening failures,
and a partial sample of screening passes were diagnosed, the sample of children
who received the diagnostic protocol were not a representative sample of all
the monolingual English speaking children sampled and screened. Therefore,
the prevalence could not be determined by simply dividing the number of SLI
children by the number diagnosed. Instead, it was necessary to determine the
prevalence of SLI in the screening failure group and the screen pass group
separately and then to combine these based upon the differential rate of screening
pass and failure. Among the screening pass children, 2.8% (31 of 108) were
found to be SLI, whereas 20.5 % (185 of 901) of the screening failures were
found to be SLI. The prevalence rate of SLI in the total screened population
was then obtained by weighting 2.8% by the proportion of children passing
the screen (.738) and likewise weighting the 23.4% by the proportion of children
failing the screen (.262). The prevalence rate of SLI in the total population
was thus estimated to be 7.42%.
A 180 item questionnaire was administered to 228 parents of children with
SLI and the parents of 860 children with normal hearing, intellectual, and
language status. This questionnaire provided information on: parental background;
maternal prenatal and perinatal health, parental exposure to tobacco, alcohol,
and other drugs; family history of speech, language and learning problems;
and several features of the child rearing environment. In addition, there
were several items that provided extended insight into certain behavioral
co-morbidities of SLI.
The principal risk factors for SLI were concentrated the background of the
parents. In comparison with the parents of the control children, the parents
of children with SLI, especially those in the urban setting, were less educated,
younger, and had greater rates of learning problems, especially learning disorders
in the mothers and speech and learning disorder in the fathers. The parents
of the children with SLI were also more likely to smoke tobacco during and
after the study child's birth than the parents of the control children. Finally,
children who were read to infrequently, and/or had little opportunity to verbally
share experiences or feelings were also at greater risk for SLI. When these
risk factors were analyzed by strata, it was found that these risk factors
were more often associated with SLI in the urban setting.