The strengths of this study are that it is the first to systematically assess sleep interventions for adults with an ID. The present study searched a wide variety of electronic databases in the search criteria with a large variety of search terms. Unfortunately, the ability to draw robust conclusions about the potential efficacy of interventions are significantly hindered by the heterogeneity of data and low quality of included studies.
This may indicate that there is a stronger evidence base for the use of melatonin that may not always be indicated. There is also a risk of publication and language biases for current literature. The evidence for assessing the effectiveness of sleep interventions for adults with an ID has significant heterogeneity and was generally of poor quality. Because of the paucity of evidence, recommendations when adults with an ID present with a sleep difficulty or sleep disorder relies on clinicians' anecdotal evidence. However, there is an opportunity to research standardised interventions using actigraphy and a standardised sleep diary in order to address heterogeneity.
In addition, medication rationalisation, epilepsy optimisation and regular health checks should be undertaken thoroughly to manage additional causes of poor sleep. Clinicians should also, where possible, screen for sleep difficulties following referrals for behaviours that challenge or mental health disorders when not responding to typical interventions. Volume 63 , Issue 5. The full text of this article hosted at iucr. If you do not receive an email within 10 minutes, your email address may not be registered, and you may need to create a new Wiley Online Library account.
If the address matches an existing account you will receive an email with instructions to retrieve your username.
Systematic Review Open Access. Shanahan Corresponding Author E-mail address: p. Paul J. Search for more papers by this author. Tools Request permission Export citation Add to favorites Track citation. Share Give access Share full text access. Share full text access. Please review our Terms and Conditions of Use and check box below to share full-text version of article. Abstract Background Current literature highlights higher prevalence rates of sleep difficulties amongst adults with an intellectual disability. Method Eight databases were searched to identify interventions for sleep difficulties amongst adults with an ID.
Results There was a notable study on heterogeneity in terms of the population, study design, intervention studied, sleep assessment and outcome assessments used. Conclusions This review highlights the need for objective measures such as actigraphy and studies with greater experimental control investigating sleep interventions in adults with ID.
Selection process Two reviewers P. Figure 1 Open in figure viewer PowerPoint. Data extraction and management The following data were collated by the first researcher P. Improvement reported in sleep duration, however, not clinically significant Hylkema and Vlaskamp 41 34 adults Netherlands 19—66 adult only Verbally reported. No P values are shown when not presented in paper. Quality assessment Quality assessment was completed by three researchers separately P. Results The initial search identified articles. Measurement of sleep difficulties A Cambridge Neurotechnology Actiwatch was used to measure duration sleeping, duration awake, sleep efficiency, number of sleep bouts, number of wake bouts, fragmentation index and hours of sleep onset latency in two studies Hare et al.
Figure 2 Open in figure viewer PowerPoint. Interventions for sleep difficulties Across the nine studies, interventions were not directly comparable. Melatonin Melatonin was used in four studies and a total of 40 participants in varying doses 2. Discussion Primary findings This systematic review aimed to identify if sleep interventions in adults with an ID improved sleep. Melatonin Melatonin was the only medication evaluated in the current review for sleep difficulties in adults with an ID.
Future research and clinical implications The goal of this study was to examine the effectiveness of sleep interventions on sleep in adults with an ID. Strengths and limitations The strengths of this study are that it is the first to systematically assess sleep interventions for adults with an ID. Conclusions The evidence for assessing the effectiveness of sleep interventions for adults with an ID has significant heterogeneity and was generally of poor quality.
Conflict of Interest The authors declare no conflicts of interest in publishing this work. Altabet S.
Mental Health Aspects Developmental Disabilities 5 , 1 — 6. Google Scholar. Crossref PubMed Google Scholar. Crossref Google Scholar. Figures References Related Information. Close Figure Viewer. Browse All Figures Return to Figure. Wodrich , Ara J.
Distilling what school practitioners really need to know, this book is learning disorders "made easy. This book features 12 case illustrations to show users how to link assessment directly to effective interventions for a wide range of problems. Chapter 6. Before his university appointment, his practice in public schools and at Phoenix Children's Hospital for more than 20 years involved evaluation of, and planning for, children with learning and developmental problems.
Finally, we attempted to review research from around the world, but our focus on history and policy is narrowly focused on the United States. We do not have good access to policy in other countries and do not always have good access to the many excellent studies completed by our international colleagues, especially in the intervention area. Even with these stipulations, the range of research covered in this book is broad and there is wide variation in the quality of the studies and syntheses we have selected for discussion.
We generally tried to select the strongest possible studies and syntheses for review. The quality of many of these pieces of information leads us to be optimistic about the continued development of both science and practice for LDs. In the future, we believe that this type of cross-disciplinary integration is essential to the development of a comprehensive model encompassing all forms of LDs, and offer this book in anticipation of continued development of an integrated understanding of LDs.
Yet LDs have traditionally been among the least understood and most debated disabling conditions affecting students Bradley et al. Indeed, the heterogeneous nature of the disability was instantiated in U. These separate types of LDs frequently co-occur with one another and with deficits in social skills, emotional disorders, and disorders of attention.
The goal of understanding LDs is to provide the most effective instruction possible in order to ameliorate the disabling effects of the conditions. However, as many researchers and practitioners have learned, identifying and understanding the nature, causes, and correlates that should be considered when teaching children with LDs is difficult. As we discuss in this book, the accumulating evidence base on LDs is now playing a more explicit and prominent role in informing instruction than ever before.
The field has progressed from simple explanations focusing on phenotypic behavioral and cognitive characteristics to more complex explanations that link cognitive, neurobiological, and instructional factors. From clinical and educational standpoints, the validity of the construct of LDs is directly linked to its ability to inform intervention decisions. As such, instruction is central to the concept of LDs as a disabling condition. If identifying students with LDs does not inform intervention and enhance communication among educators providing the instruction, then the concept would be virtually meaningless—except as a legal definition of a group of people with disabilities requiring civil rights protection.
To understand how these alternative perspectives have evolved, this chapter examines the historical underpinnings of LDs. These commentaries indicate that the field of LDs developed in response to two major needs. First, the emergence of the field was linked to a need to understand individual differences in learning and performance among children and adults displaying specific deficits in spoken or written language, while maintaining integrity in overall adaptive functioning. Unexpected patterns of strengths and specific weaknesses in learning were first noted and studied by physicians and psychologists, thus giving the biomedical and psychological orientation that has always characterized the field of LDs.
Second, the LD movement developed as an applied field of special education driven by social and political forces, and from a need to provide services to youth whose learning characteristics were not being adequately ad- History of the Field 11 dressed by the educational system. Each of these historical contexts is reviewed briefly. In describing the characteristics of one patient with brain damage, Gall recorded the following: In consequence of an attack of apoplexy, a soldier found it impossible to express in spoken language his feelings and ideas.
His face bore no signs of a deranged intellect. His mind esprit found the answer to questions addressed to him and he carried out all he was told to do; shown an armchair and asked if he knew what it was, he answered by seating himself in it. He could not articulate on the spot a word pronounced for him to repeat; but a few moments later the word escaped from his lips as if voluntarily.
It was not his tongue, which was embarrassed; for he moved it with great agility and could pronounce quite well a large number of isolated words. His memory was not at fault, for he signified his anger at being unable to express himself concerning many things, which he wished to communicate. It was the faculty of speech, alone, which was abolished. Hammill postulated that Gall noted that some of his patients could not speak but could produce thoughts in writing, thus manifesting a pattern of relative strengths and weaknesses in oral and written language. In addition, Gall established that such patterns of strengths and weaknesses were a function of brain damage, and that brain damage could selectively impair one particular language capability but not affect others.
Early Neurology and Acquired Language Disorders A number of other medical professionals also began to observe and report on patients demonstrating intraindividual strengths and weaknesses that included specific deficits in linguistic, reading, and cognitive abilities. The effects of a lesion in this area of the brain were highly consistent in right-handed individuals and did not appear to affect receptive language ability listening or other nonlanguage functions e.
Wernicke reported that a complex function such as receptive language could be impaired within an individual who did not display other significant cognitive or linguistic dysfunctions. Hence, the concept of intraindividual differences in information processing was born, primarily using observations and clinical studies with adults with specific brain damage.
In the late s and early s, additional cases of unexpected cognitive and linguistic difficulties within the context of otherwise normal functioning were reported. These cases were unique because they did not seem to have the same neurological characteristics as acquired disorders of language occurring with impairment of sensory or motor functions. Kussmaul described a patient who was unable to read despite having sufficient intellectual and perceptual skills.
Additional reports by Hinshelwood , , Morgan , and others Bastian, ; Clairborne, distinguished a specific type of learning deficit characterized by an inability to read against a background of normal intelligence and adequate opportunity to learn. Hinshelwood described a year-old youngster as follows: The boy had been at school three years and had got on well with every subject except reading.
He was apparently a bright and in every respect History of the Field 13 an intelligent boy. He had been learning music for a year and had made good progress in it. In all departments of his studies where the instruction was oral he had made good progress, showing that his auditory memory was good.
He performs simple sums quite correctly, and his progress in arithmetic has been regarded as quite satisfactory. He has no difficulty in learning to write. His visual acuity is good. Orton and the Origins of Dyslexia During the s, Samuel Orton extended the study of reading disabilities with clinical studies designed to test the hypothesis that reading deficits were a function of a delay or failure of the left cerebral hemisphere to establish dominance for language functions.
Orton was the first to stress that reading disabilities manifested at a symbolic level appeared to be related to cerebral dysfunction rather than a specific brain lesion as postulated by Hinshelwood and others and could be identified among children with average to above-average intelligence. This work built on earlier attempts to understand the behavioral difficulties of children who subsequently were described as hyperactive. Strauss and Werner expanded this concept in research involving children with mental retardation.
They were particularly interested in comparing the behavior of children whose retardation was associated with known brain damage, with that of children whose retardation was not associated with neurological impairment but was presumably familial in nature. Strauss and Lehtinen reported that children with mental retardation and brain injury manifested difficulties on tasks assessing figure—ground perception, attention, and concept formation in addition to hyperactivity.
However, children without brain damage but with mental retardation performed in a manner similar to children who were not mentally impaired and were less likely to show behavioral overactivity. When no mental retardation exists, the presence of psychological disturbances can be discovered by the use of some of our qualitative tests for perceptual and cognitive disturbances.
Although the [physical] criteria may be negative, whereas the behavior of the child in question resembles that characteristic for brain injury, and even though the performances of the child on our tests are not strongly indicative of brain injury, it may still be reasonable to consider a diagnosis of brain injury. Torgesen concluded that three concepts emerging from the Straussian movement provided a rationale for the development of the field of LDs separately from other fields of education: 1 Individual differences in learning could be understood by examining the different ways that children approach learning tasks the processes that aid or interfere with learning ; 2 educational procedures should be tailored to patterns of processing strengths and weaknesses in the individual child; and 3 children with deficient learning processes may be helped to learn normally by employing teaching methods that focus on their processing strengths rather than their weaknesses.
Expanding on this list, Kavale and Forness included 1 The locus of an LD is within the affected individual, and thus represents a medical disease model; 2 LDs are associated with or caused by neurological dysfunction; 3 the academic problems observed in children with LDs are related to psychological processing deficits, most notably in the perceptual—motor domain; 4 the academic failure of children with LDs occurs despite the presence of normal intelligence; and 5 LDs cannot primarily be due to other handicapping conditions.
Cruickshank, Myklebust, Johnson, and Kirk and the Concept of LDs Among the most significant behavioral scientists involved in the early conceptualization and study of LDs were William Cruickshank, Helmer Myklebust, Doris Johnson, and Samuel Kirk, all of whom propelled the field away from a focus on etiology toward an emphasis on learner characteristics and educational interventions to address learning deficits. Helmer Myklebust and Doris Johnson at Northwestern University conducted numerous studies of the effects of different types of language and perceptual deficits on academic and social learning in children.
In this group, I do not include children who have sensory handicaps such as blindness, because we have methods of managing and training the deaf and blind. I also excluded from this group children who have generalized mental retardation. This movement was based largely on the arguments of Kirk and others that children with LDs 1 had different learning characteristics than children diagnosed with mental retardation or emotional disturbance; 2 manifested learning characteristics that resulted from intrinsic i.
Note that in this insightful definition, no mention is made of intelligence. Exclusionary conditions are identified on the basis of differential intervention needs, not simply defining LDs in terms of what conditions are not LDs. What is interesting is that the field received its initial momentum on the strength of clinical observation and advocacy. In fact, in the United History of the Field 17 States, the majority of scientific advances are typically stimulated by vocal critics of the educational or medical status quo. It is rare that a psychological condition, disease, or educational problem is afforded attention until political forces are mobilized by parents, patients, or other affected individuals expressing their concerns about their quality of life to their elected officials.
Clearly, this was the case in the field of LDs, in which parents and child advocates successfully lobbied Congress to enact legislation in through the Education of the Handicapped Act Public Law This law authorized research and training programs to address the needs of children with specific LDs Doris, The diagnostic concept of LDs gained significant momentum during the s and s.
As Zigmond explained, the proliferation of children diagnosed as having LDs during these two decades was related to multiple factors. The fact that youngsters with LDs displayed strong intelligence gave parents and teachers hope that learning difficulties could be surmounted, given that the right set of instructional methods, conditions, and settings could be identified. Advocacy efforts fueled a series of consensus conferences, two of which are noteworthy: one on MBI and the other on LDs. Both attempted to identify a single overarching diagnostic category that could define the disabilities widely believed to hamper the educational and behavioral performance of many children.
The first significant effort involved the development of a definition of MBI in These deviations may manifest themselves by various combinations of impairment in perception, conceptualization, language, memory, and control of attention, impulse, or motor function. Clements, , pp. As noted earlier, this definition stipulated that brain dysfunction could be identified solely on the basis of behavioral signs. To the educational community, MBD was closely connected to a medical model and implied that psychologists and physicians would have to work in schools in order to make a diagnosis.
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Others found the concept fuzzy and too broad Rutter, These symptoms ranged from difficulties with academic skills to aggressive, acting-out behavior. However, both require intervention. Heritability, neurobiological correlates, and intervention needs are different, so unifying them as a single syndrome did not facilitate research or practice.
History of the Field 19 U. Federal Definition of LDs Not surprisingly, the development of the definition of MBD led to reactions among educators and other professionals working in schools. In , the U. The term includes such conditions as perceptual handicaps, brain injury, minimal brain dysfunction, dyslexia, and developmental aphasia. The term does not include children who have learning disabilities, which are primarily the result of visual, hearing, or motor handicaps, or mental retardation, or emotional disturbance, or of environmental, cultural, or economic disadvantage.
Office of Education, , p. Etiological terms were dropped and replaced by educational descriptors. However, the pivotal importance of this definition is that it continues to serve as the U. It has persisted through a series of parental and professional advocacy efforts that led to the provision of special education services for children with LDs. This occurred initially through the Learning Disabilities Act. This definition has endured despite the fact that it does not specify any inclusionary criteria for LDs.
It essentially says that LDs are heterogeneous, reflect problems with cognitive processing, and are not to be commingled with other disorders that represent exclusionary conditions. In a sense, LDs became legitimized and codified in U. In response to this problem, the U. Office of Education published recommendations for procedures for identifying LDs that included the notion of a discrepancy between IQ and achievement as a marker for LDs, as follows: a severe discrepancy between achievement and intellectual ability in one or more of the areas: 1 oral expression; 2 listening comprehension; 3 written expression; 4 basic reading skill; 5 reading comprehension; 6 mathematics calculation; or 7 mathematic reasoning.
The child may not be identified as having a specific learning disability if the discrepancy between ability and achievement is primarily the result of: 1 a visual, hearing, or motor handicap; 2 mental retardation; 3 emotional disturbance; or 4 environmental, cultural, or economic disadvantage. However, researchers, practitioners, and the public continue to assume that such a discrepancy is a marker for specific types of LDs that are unexpected and categorically distinct from other forms of underachievement. But the impact of IQ—achievement discrepancy was clearly apparent in the regulations concerning LD identification in the and reauthorizations of IDEA.
The statute has maintained the definition of LDs formulated in the meeting, and the regulations maintained the procedures until the reauthorization. Other formal attempts to tighten the federal definition of LDs have not fared significantly better, as can be seen in the revised definition produced by the National Joint Committee on Learning Disabilities NJCLD, ; see also Hammill, : Learning disabilities is a general term that refers to a heterogeneous group of disorders manifested by significant difficulty in the acquisition and use of listening, speaking, reading, writing, reasoning, or mathematical abilities.
These disorders are intrinsic to the individual, presumed to be due to central nervous system dysfunction, and may occur across the life span. Problems in self-regulatory behavior, social perception, and social interaction may exist with learning disabilities but do not by themselves constitute a learning disability. Although learning disabilities may occur concomitantly with other handicapping conditions for example, sensory impairment, mental retardation, social and emotional disturbance or with extrinsic influences such as cultural differences, insufficient or inappropriate instruction , they are not the result of these conditions or influences.
These types of definitions cannot be easily operationalized or empirically validated and do not provide clinicians, teachers, or researchers with useful information to enhance communication or improve predictions. There are no inclusionary criteria, and the definition is based on exclusion. Given this state of the field, many scholars have called for a moratorium on the development of broad definitions and advocate definitions that address LDs only in terms of coherent and operational domains.
Research investigations must define groups specifically in terms of the domain of deficit reading disability, arithmetic disability. The extent of co-occurrence of these dysfunctions then becomes an empirical question, not something decided a priori by definition practices. These definitions implicitly support the heterogeneity and exclusion components of most definitions. Interestingly, the definitions invoke IQ—achievement discrepancy as an inclusionary criterion. Regardless of whether one approaches the task of defining LDs in a general fashion as has been traditionally done at the federal level, or whether one seeks to define domain-specific LDs e.
The logic underlying the development of such a classification system is that identification, diagnosis, treatment, and prognosis cannot be addressed effectively until the heterogeneity across and within domain-specific LDs is addressed, and until subgroups are delineated that are theoretically meaningful, reliable, and valid. Of utmost importance is the validity of the three classification hypotheses discrepancy, heterogeneity, exclusions implicit in most definitions of LDs. Congress passed statutes that permitted alterations of the regulations, indicating specifically that 1 states could not require districts to use IQ tests for the identification of students for special education in the LD category, and 2 states had to permit districts to implement identification models that incorporated response to instruction RTI IDEA, In addition, the statute clearly indicated that children could not be identified for special education if poor achievement was due to lack of appropriate instruction in reading or math, or to limited proficiency in English.
What is noteworthy is that the statute and regulations are based on the converging scientific evidence bearing on the limited value of IQ—achievement discrepancies in identifying LDs, while at the same time underscoring the value of RTI in the identification process. Although issues surrounding the validity of IQ—achievement discrepancies and RTI are discussed in detail in Chapter 3, the regulations relevant to LDs are summarized here.
In essence, regulations indicate that states: 1. May not require local education agencies LEAs to use a discrepancy model for determining whether a student has LDs. Must permit the use of a process that determines if the student responds to scientific research-based intervention.
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May permit other alternative research-based procedures. Although a number of advocacy and practitioner groups questioned specific provisions of the regulations, what is encouraging is that all organizations have acknowledged the critical importance of using research to guide policies and practices concerning students with LDs, which is clearly reflected in the IDEA statutes and regulations. Equally significant in the new statute and regulations is the more explicit recognition that LDs should not be identified in the absence of evidence of appropriate instruction. The statute indicates that LDs may not be identified if the cause of poor achievement is inadequate instruction in reading or math, or limited proficiency with English by requiring: 1.
Evidence of appropriate instruction in reading and math in general education. This information must be provided to parents and included in team decisions determining whether the child has an LD, that the LD is a disabling condition, and that special education services are warranted. Thus, the IDEA statute moves toward the accumulating research base on LDs by reducing the focus on IQ tests and emphasizing the critical role of instruction both for preventing LDs and for their identification. LDs constitute a diagnostic category of interest to clinical practice, law, and policy.
In many respects, however, LDs have been legitimized and codified in public law on the basis of what they are not, that is, through a focus on definition by exclusion. Moreover, the concept of LDs is based on what is now a century of attempts to define it as an overarching classification applicable to a wide segment of childhood difficulties involving learning and behavior. Only in the past 30 or so years have systematic research efforts emerged that make progress toward understanding the causes, developmental course, treatment conditions, and long-term outcomes of LDs a reality.
Despite significant research advances, many of these efforts have not led to more precise definitions and interventions for those with LDs. However, the revisions in the reauthorization of IDEA could ensure that policies and practices will be based on converging scientific evidence. If the field of LDs is to progress and result in positive outcomes, it has little choice. The reification of historically unsupported assumptions about LDs that collapse under scientific scrutiny may hinder the successful application of what we have learned from the significant advances in research that have occurred over the past 30 years.
This is unfortunate. The groups of advocates who successfully implemented essential educational reforms legitimizing the concept of LDs and helped make a systematic research program possible may be continuing to support components of the definition that are outdated, indefensible, and not aligned with research.
In doing so, they may be promulgating identification and intervention practices that are not effective, making it difficult to implement practices that have emerged from research Fletcher et al. This problem emerged early in U. Furthermore, this lack of clarity has interfered with the provision of accommodations for high-stakes accountability and college aptitude tests, the selection of people with LDs for research studies, individual eligibility for insurance, social security, and other entitlements, and the development of specialized interventions.
At the same time, these early efforts sought to differentiate LDs from their historical antecedents, epitomized by the MBD hypothesis. Despite more than a century of efforts, definition issues remain inadequately resolved, though some progress has been made. There is no better arena for highlighting definition issues than educational policy. As we discussed in Chapter 2, in the reauthorization of the U. On the basis of their review of the converging scientific evidence, both the U. Despite the evidentiary support for this change in policy, substantial resistance to policy modifications have come from individuals and groups concerned that changes in current practice could lead to reduced services for individuals with LDs.
As with any change in educational policy, such resistance was not unexpected. At the heart of the definition problem is a lack of understanding of the criteria by which different disorders are classified so that the resulting categories have both internal and external validity. For this reason, in this chapter we approach the definition issue from a classification perspective, reviewing evidence for the reliability and validity of four different models: aptitude—achievement discrepancy, low achievement, intraindividual differences, and response to instruction RTI.
We also review the evidence for the validity of the different exclusionary conditions observed in most definitions of LDs. Classifications are systems that permit a larger set of entities to be partitioned into smaller, more homogeneous subgroups based on similarities and dissimilarities in attributes thought to define different aspects of the phenomenon of interest. When entities are assigned, or identified, to subgroups, the process represents an operationalization of the definitions emerging from the classification. Diagnosis or identification occurs when the operational definitions are used to determine membership in one or more subgroups.
Although the terminology describes groupings, we define groupings as decisions made about how individuals are related on correlated dimensions that define the subgroups. The decisions can appear arbitrary and are subject to measurement error. Thus, it is critical to formally assess the validity and reliability of the subgrouping.
Valid classifications do not exist solely because subgroups can be created. Rather, the subgroups making up a valid classification can be differentiated according to variables i. Validity, however, hinges on evidence that the classification is not dependent on the method of classification, can be replicated in other samples, and permits identification of the majority of entities of interest i. Reliable and valid classifications facilitate communication, prediction, and treatment, although different classifications may be better for some of those purposes than others Blashfield, For LDs, classification occurs in identifying children as needing intervention; as learning disabled or as typically achieving; as learning disabled as opposed to mentally retarded or having ADHD; and, within LDs, as reading rather than math impaired.
By virtue of exclusionary criteria, LDs are hypothesized to represent a subgroup of people with unexpected underachievement. These levels of classification represent hypotheses that should be evaluated for the reliability of the hypothetical model and for validity by reference to variables that are different from those used to establish the classification and assign individuals to subgroups.
The first is that, as a construct, LD represents an unobservable latent variable that does not exist apart from attempts to measure it. The second involves the dimensional nature of LDs i. LD Is an Unobservable Construct We alluded to the latent construct representing LDs in Chapter 2, noting the efforts to identify a group of children as underachieving despite the absence of circumstances that produce low achievement. These efforts represented attempts to assess unexpected underachievement, typically conceptualized as individuals unable to master academic skills despite the absence of known causes of poor achievement sensory disorder, mental retardation, emotional disturbances, economic disadvantage, linguistic diversity, inadequate instruction.
Many efforts at definition and identification have been attempts to measure this attribute, which epitomizes the LD construct. Thus, the appeal of aptitude—achievement discrepancies is the relatively simple task of assessing IQ and academic achievement to determine if a discrepancy exists between the two domains.
Unfortunately, the evidence for this hypothesis is weak and the measurement of unexpected underachievement is not a simple task. By establishing definitions of LDs and using these definitions to assess people for the presence or absence of the unobservable latent variable, all efforts at measurement will be imperfect and inconsistent because of differences in how the construct is measured, leading to differences in who is identified with an LD. Each of these observable measures indicates, albeit imperfectly, the latent variable of LD.
The measurement is imperfect because no single measure captures all the components of the construct and each measurement contains a certain amount of error. The critical issue is the effect of these imperfect measurements on the reliability and validity of the overarching classification that is the basis for identifying LD. This is the essence of classification research.
Most of the research on LDs, particularly those affecting reading, shows that they occur along a continuum of severity rather than presenting as an explicit dichotomous category delineated by clear cut-points on the achievement distribution. Shaywitz, Escobar, B. This conclusion is not without controversy. The studies by Miles and Haslum and Wood and Grigorenko do not provide enough details for evaluation of their findings.
Shaywitz, As these are dimensional traits that exist on a continuum, there would be no expectation of natural cut-points that differentiate individuals with LDs from those who are underachievers, but not identified with LDs; the distribution is simply a continuum of severity S.
Shaywitz et al. If we simply dealt with the average performances of groups with and without LDs, as in research, the dimensional nature of LDs and the imperfection of measurements of the construct would not be a major problem because the errors of measurement would be reflected in the variability around the mean. However, it is necessary to identify individuals who have or do not have LDs and we rarely talk of degrees of LD except in terms of severity, which is also a continuous concept , making it necessary to categorize what are inherently normal distributions of some attribute serving as an indicator of LD e.
In research, LD is commonly defined according to a cut-point e. The use of a cut-point, particularly when the score is not criterion referenced and the score distributions have been normalized, is a major problem when the underlying attribute is continuous. The problem occurs in part because of the measurement error of any test.
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Because of measurement error, any cut-point will lead to instability in the identification of specific individuals for the category. Scores will fluctuate around the cut-point with repeat testing, even for a decision as straightforward as demarcating low achievement or mental retardation Francis et al. This fluctuation is not the result of repeat testing, nor is it a matter of selecting the ideal cut-point. Thus, it is common in the identification of LDs to add other criteria, such as an absence of other disorders that cause low achievement, to try to improve the accuracy of the assessment of the latent construct.
Subdividing a normal distribution to create groups has been criticized in the measurement literature Cohen, The group structure is often arbitrary when the distributions are dimensional in nature and may constrain the variability within groups and reduce the range of measurement.
The subdivision thus distorts the relative importance of the underlying dimensions to performance on other measures, leading to reduced power in statistical comparisons, as well as inaccurate results due to the failure to allow for the correlation between different dimensions.
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Because individuals around the cut-point are similar, the error around the cut-point is not a major issue. The amount of error related to the cutpoint could influence the size of the effect, but this issue also involves the correlations of the dependent and independent variables. If the effect size is the major focus of the research, there is little reason for the use of a subdivision of the measured attribute, and any questions could be addressed with correlation methods Stuebing et al.
However, it is necessary to determine the severity of the LD to identify those in need of services, accommodations, and better treatment from society. Therefore, LDs can never be defined solely on the basis of cut-points on psychometric procedures, particularly if the attribute is measured only once, which magnifies the effect of the error of measurement Francis et al. Many of the issues involving different models for identifying children with LDs reflect confusion about the relation of classification, definition, and identification.
The relation is inherently hierarchical in that the definitions derived from a classification yield criteria for identifying members of the subcomponents of the classification. Definitions of LDs originate from an overarching classification of childhood disorders that differentiate LDs from mental retardation and various behavior disorders, such as ADHD.
This classification yields definitions and criteria based on attributes that distinguish LDs from mental retardation and ADHD. These criteria can be used to identify children as members of different subgroups within the classification model. In evaluating these models, we assumed that a valid classification must identify individuals who represent a subgroup with unexpected underachievement. The pattern of differences among low achievers identified as learning disabled or not learning disabled in each model should lead to a unique set of characteristics in those identified as learning disabled.
Assessing the validity of the classification should evaluate how well the definition produces a unique group of low achievers when variables are used that were not part of the approach to identification. Aptitude—Achievement Discrepancy Although the most common approach to determining aptitude—achievement discrepancy is the identification of a discrepancy between the results of an IQ test and a test of achievement, there is disagreement as to which IQ and achievement tests should be used.
We focus initially on discrepancies between a composite measure of IQ and reading achievement, typically word reading.
David L. Wodrich
We then review issues related to the proposed use of a verbal IQ measure, nonverbal IQ measure, and non-IQ measure, such as listening comprehension. We also review other domains that do not necessarily involve LDs, but for which similar models have been proposed. Our focus is on whether external variables validate the hypothesized classification. This is an assumption that has its origins in the earliest development of IQ tests and one that has been debated since their inception Kamin, It is beyond the scope of this book to review this debate, except to note that this assumption is inherent in the use of any aptitude measure and widely questioned.
These studies, reviewed by Aaron , Siegel , Stuebing et al.