The Bell Curve as a Diagnosis Tool.

One of the tools that was used to diagnose Katie's ASD was the bell curve.   Thought I would post some information about the sort of testing that helps professionals make these diagnosis.  Katie's IQ is well with in normal range but her adaptive functioning has significant deficits. This post is not to imply that your child has any level of retardation-not at all.  However, understanding the Bell Curve also helped me to understand how the ASD may impact her future as far as integrating into society.  Section #2 may seem that it relates to your child more than section #1 does but section #1 includes a lot of explanation of the test itself.

1) Intellectual Functioning and Intelligence Tests

All definitions of mental retardation require that the individual have abnormally low intellectual functioning. The two most widely used definitions come from the American Psychiatric Association and the American Association on Mental Retardation (AAMR). The American Psychiatric Association (APA) describes mental retardation in the Diagnostic and Statistical Manual, 4^th Edition, Text Revised (DSM).

In general, the DSM codifies psychiatric diagnoses with sets of specific diagnostic criteria. The diagnoses are not tied to an etiology and the criteria are usually broad enough to allow for considerable heterogeneity among people sharing a diagnosis. In the DSM, a diagnosis of mental retardation requires an IQ of “approximately 70 or below” as well as deficits in two areas of adaptive functioning (described below). The DSM definition further codes mental retardation by severity. Mild mental retardation has an IQ range of 50-55 to approximately 70, moderate mental retardation has an IQ range of 35-40 to 50-55 and severe mental retardation has an IQ range of 20-25 to 35-40. Persons with an IQ below 20 or 25 are diagnosed as profoundly mentally retarded. The DSM allows for the diagnosis of “Mental Retardation, Severity Unspecified” when a person appears to have deficits consistent with mental retardation but is too impaired or uncooperative to be tested. There is also a diagnosis of “Borderline Intellectual Functioning” that is associated with IQ in the range of 71-84 (APA, 2000).

Among the 26 states that define mental retardation in statutes prohibiting the death penalty for people with mental retardation, ten generally follow the APA’s diagnostic criterion and specify an IQ of 70 or below. (Illinois is an exception, with a specified IQ of 75 or below). The other sixteen states and the federal government define mental retardation with language adapted from the AAMR’s 1992 definition: “Significantly subaverage general intellectual functioning” with emphasis on deficits in adaptive functioning (AAMR, 1992) and do not specify an IQ score.

Intelligence tests are standardized tests that measure a person’s intellectual capacities and compare that measurement to a population’s scores. Standardization is accomplished by training the professionals who administer the tests (usually psychologists with a clinical doctoral degree) to administer the same tests under the same conditions every time. Persons taking the tests should be rested, comfortable and alert. The test-taking environment should be quiet and without interruptions. The test is administered in a specific order and each part of the test is timed. The tests are scored and interpreted in a standardized fashion as well.

The test is normed with the average score defined to be 100 and people are compared with others in the same age range. In a random population of individuals, IQ scores will be distributed in a “normal” distribution or a bell shaped curve and the variability of scores is predictable. Two thirds of the population will fall within one standard deviation from the mean score of 100. The standard deviation (the spread of variation from the mean) is about 15 for intelligence tests. Two thirds of the population will fall in the range of +/- 1 standard deviation with scores between 85 to 115. Two standard deviations will capture 95% of a population. This corresponds to the IQ range of 70 to 130. The APA’s Council on Psychiatry and the Law defined “significant limitation in intellectual functioning” to be two standard deviations below the mean, similar to the DSM-IV TR criteria of an IQ of approximately 70. The term “approximate” reflects the standard error of intelligence tests.

There are several versions of intelligence tests which vary in length and target population (children or adults). The most commonly used are the Wechsler tests that are known by acronyms such as the WISC-lV (for children aged 6 to 16 years) and the WAIS-lll (for ages 16 through adulthood). The WAIS is in its third edition; the WISC recently was updated to a fourth edition. Because the Wechsler tests are so widely used, the reliability and validity are better than lesser used tests. The WAIS-III is made up of fourteen sub-tests, Seven subtests contribute to the verbal subscale: information, comprehension, arithmetic, similarities, vocabulary, digit span, and letter-number sequencing. Seven subtests contribute to the performance subscale: picture completion, digit symbol-coding, block design, matrix reasoning, picture arrangement, symbol search, and object assembly.  The test also provides a composite, single full-scale IQ score based on the combined scores.

Although intelligence tests are standardized, there are influences that may distort an individual’s score. Cultural and educational factors are thought to influence measurement of intelligence. The Flynn effect is another potential problem. This is the finding that IQ scores tend to rise over time. In order to keep the average score at 100, the tests are renormed periodically, making them slightly harder. The renorming of intelligence tests might distort an individual’s intellectual abilities at the end and beginning on a new edition of the test. At the end of a test edition cycle, an individual with borderline or mild mental retardation may score five or more points higher on the older test than on the newer, more difficult edition (Kanaya, 2003). An additional problem of IQ scores is the possible increase in scores after repeated testing because the person taking the test has had practice with the tasks. Practice effects are more prominent when retesting occurs within a six to twelve month period and primarily affects the performance subscale.

                       

2) Adaptive Functioning

In addition to the criterion of decreased intellectual functioning, the definitions of mental retardation all require evidence of problems in adaptive functioning prior to age 18. The APA definition in the DSM describes this as “deficits or impairments in present adaptive functioning (i.e. the person’s effectiveness in meeting the standards expected for his or her age by his her cultural group).Deficits in two or more of the following areas are required for the diagnosis: communication, self-care, home living, social/ interpersonal skills, use of community resources, self-direction, functional academic skills, work, leisure, health and safety” (APA, 2000). The AAMR definition includes the subaverage intellectual functioning described above plus limitations in conceptual, social and practical skills during the time period prior to age eighteen. (AAMR, 2002). The AAMR emphasizes that problems in adaptation are directly related to the demands of the environment.  

The AAMR has published an assessment manual which matches skill areas in conceptual, social and practical domains with assessment instruments such as the Adaptive Behavior Scale. The Vineland Adaptive Behavior Scale is another widely used test of adaptive skills. These scales measure a wide range of abilities such as feeding self, the use tools or utensils, the use of the toilet, the ability to handle money and the ability to follow current events. Although the instruments are standardized, there is a greater role for clinical judgment in assessing adaptive behavior than in measurements of IQ.

The clinical definitions of mental retardation require onset of decreased intellectual and adaptive functioning prior to age 18, reflecting a problem in development. Some state statutes specify a different age such as 22 in Indiana and Maryland and other states do specify any age. It is likely that some individuals suspected of having mental retardation did not receive IQ tests and adaptive functioning assessments prior to age 18. In these cases, the assessment must include a careful review of old records for evidence of lower levels of functioning and testing of the individual in the correctional environment.

Evidence of decreased intellectual and adaptive functioning before age 18 may be found in school records, social services records, juvenile justice reports, military records, employment records and pediatric records. It is unlikely, for example that a mildly mentally retarded person would score near the average range of a school achievement test. It is more likely that such an individual would be identified as “slow” some time in early gradeschool, score very low on school achievement tests, require special education classes and an Individualized Education Plan (IEP). If IQ and disability assessments before age 18 are available, the tests and standards may be different from those in current practice. It is valuable to have interviews and reports from teachers, parents and other caregivers although retrospective information is less reliable than contemporaneous documentation. An evaluation of past functional abilities should include multiple sources (Bonnie, 2004The assessment of functional adaptation in the correctional setting is problematic. Areas of functional disability included in the definitions may not have a relevant counterpart in jails or prisons. For example community resources, leisure skills and self direction have little or no application in an institutional environment. It is likely that a mentally retarded person will show better adaptive functioning in the more structured correctional environment than in general society.

References:

American Association on Mental Retardation, (2002) Mental Retardation: Definition, Classification, and Systems of Supports Ruth Luckasson Ed., 10^th Ed. AAMR’s website is www.aamr.org.

American Psychiatric Association (2000) Diagnostic and Statistical Manual of Mental Disorders, 4^th edition, text revised.  Washington D.C..

Bonnie, R.J. (2004) The American Psychiatric Association’s resource document on mental retardation and capital sentencing: Implementing Atkins v. Virginia. J Am Acad Psychiatry Law 32:304-8.

Finlay, W.M. and Lyons, E. (2002) Acquiescence in interviews with people who have mental retardation Mental Retardation 40(1) 14-29.

 Hurley, K.E. and Deal, W.P. (2006) Assessment instruments measuring malingering use with individuals who have mental retardation: Potential problems and issues.  Mental Retardation 44(2) 112-119.

Kanaya, T., Scullin, M.H. and Ceci, S.J. (2003) The Flynn effect and U.S. policies: the impact of rising IQ scores on American society via mental retardation diagnoses. Am Psychol 58(10): 778-90.