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Mental Retardation

Mental Retardation is defined as below-average general intellectual function with associated deficits in adaptive behavior that occurs before age 18. It is characterized by significantly sub-average intellectual functioning, existing concurrently with related limitations in two or more of the following applicable adaptable skill areas: communication, self-care, home living, social skills, community use, self direction, health and safety, functional academics, leisure and work.

Mental retardation has been classified by the degree or level of intellectual impairment, as measured by an IQ test. The most widely used classification method consists of four levels of mental retardation according to the range of IQ scores shown in the table below.

Level Intelligence Test Score
Mild retardation 50-55 to approx. 70
Moderate retardation 35-40 to 50-55
Severe retardation 20-25 to 35-40
Profound retardation Below 20-25

There are numerous causes of mental retardation, but a specificity is determined in only 25% of the cases. Failure to adapt normally and grow intellectually may become apparent early in life or, in the case of mild retardation, not become recognizable until school age or later. An assessment of age-appropriate adaptive behaviors can be made by the use of developmental screening tests. The failure to achieve developmental milestones is suggestive of mental retardation. A family may suspect mental retardation if motor skills, language skills, and self-help skills do not seem to be developing in a child or are developing at a far slower rate than the child’s peers. The degree of impairment from mental retardation has a wide range from profoundly impaired (5%) to mild or borderline retardation (80-90%). Less emphasis is now placed on degree of retardation and more on the amount of intervention and care required for daily life.


Developmental Assessment Tools

Bayley Scales of Infant Development:
The Bayley Scales of Infant Development measure mental and physical, as well as emotional and social, development. The test, which takes approximately 45 minutes, is administered individually by having the child respond to a series of stimuli. The Mental Scales, which measure intellectual development, assess functions such as memory, learning, problem-solving ability, and verbal communication skills. The Motor Scales evaluate the child's ability to sit and stand, perform other activities requiring coordination of the large muscles (gross motor skills), and perform more delicate manipulations with fingers and hands (fine motor skills). Finally, the Infant Behavior Record (IBR) assesses the child's social and emotional development through a standardized description of his or her behavior during the testing session. Scores are measured against norms for each of the 14 different age groups. Often, the Bayley scales are used to determine whether a child is developing normally and provide for early diagnosis and intervention in cases of developmental delay, where there is significant tardiness in acquiring certain skills or performing key activities. Additionally, they can be used to qualify a child for special services and/or demonstrate the effectiveness of those services.

Differential Ability Scales (DAS):
The Differential Ability Scales is an individually administered battery of cognitive and achievement tests for children and adolescents aged 2 years, 6 months through 17 years, 11 months. It is divided into three levels:

  • Lower Preschool (ages 2 years, 6 months through 3 years, 5 months)
  • Upper Preschool (aged 3 years, 6 months through 5 years, 11 months)
  • School-Age (6 years, 0 months through 17 years, 11 months)
The DAS was designed to measure specific, definable abilities and to provide interpretable profiles of strengths and weaknesses. The DAS is considered suitable for use in any setting in which the cognitive abilities of children and adolescents are to be evaluated, although many of the DAS subtests are not appropriate for students with severe sensory or motor disabilities. The cognitive battery focuses on reasoning and conceptual abilities and provides a composite standard score, the General Conceptual Ability (GCA) score.

The DAS contains a total of 20 subtests grouped into Core Cognitive, Diagnostic, or Achievement tests. The Core Cognitive subtests are those used to compute the GCA and cluster scores, while the Diagnostic subtests are those considered important and useful in the interpretation of an individual's strengths and weaknesses, but which do not assess "complex mental processing" well. It has a built-in mechanism for assessing significantly delayed children who are over the age of 3˝ years. It can also provide information comparable to other similar instruments in about half the time.

Wechsler Preschool and Primary Scale of Intelligence-Revised (WPPSI-R)
The WPPSI-R is an individually administered, norm-referenced, standardized test for children ages 3 years to 7 years 3 months to assess intelligence. It is organized into 2 groups of subtests, perceptual-motor (performance) and verbal. These yield the Performance scale and Verbal scale IQ scores. These two IQ scales combine to yield a Full Scale IQ, with a mean of 100 and a standard deviation of ±15. The WPPSI-R has one group of perceptual-motor (Performance) subtests and a second group of Verbal subtests. The child responds to the Performance subtests with motor responses such as pointing, placing or drawing. The Verbal subtests require verbal responses from the child. The starting points and discontinuing rules are given in the directions for each subtest. The subtests are customarily started according to the child's age. The discontinue rule is usually a certain number of consecutive failures. The WPPSI-R has 12 subtests, 2 of which are optional. This instrument cannot be used with severely disabled children (IQ's below 40) and, with younger children, may need to be administered over two sessions due to the length of time required to complete the assessment.

Wechsler Intelligence Scale for Children-III (WISC-III)
This intelligence test is the universal standard which School Psychologists use most often. This version of the Wechsler is standardized for children from age 6 to 16. The test, itself, is divided into two main sections. The Verbal Scale measures how well children are able to express themselves verbally and how well they are able to understand what is being said to them. The Performance Scale measures the nonverbal areas of being able to perceive spatial relationships; such as in putting puzzles together, and being able to transfer visual information rapidly. Using test interpretation, the three I.Q. scores and the specific pattern of strengths and weaknesses indicate how well the child is able to learn and whether there are any specific learning disabilities. This information is then used to predict at what academic level the child should be functioning. In this way, diagnoses of learning impairments are possible.

Wechsler Adult Intelligence Scale - Revised (WAIS-R)
One of the most frequently used tests of adult intelligence, it is based upon a series of subtests with two general categories of items, verbal and performance. Verbal items deal with general information, vocabulary, arithmetic tests, comprehension, similarities, analogies, etc.; performance items deal with picture arrangement and completion, block designs, spatial relations, and the like. The test was revised in 1981 and is familiarly known as the WAIS-R, pronounced 'waiss-are.' The WAIS-R covers an age range of 16 years, 0 months to 74 years, 11 months.

Stanford-Binet Intelligence Scale 4th Ed.
This intelligence test is also considered to be a standard tool of many School Psychologists. This test has been fairly recently revised and now provides multiple I.Q. scores (called S.A.S.'s) instead of a single I.Q. score, as before. In addition to being able to measure the verbal and nonverbal areas of a child's development, the Binet also provides a quantitative score, measuring the child's mathematical reasoning, and a memory score, measuring the child's short-term memory. (While the Wechsler scales also have subtests which measure these areas, they do not provide I.Q. scores isolating these abilities.) The materials in this test are very appealing to children. The child has little chance to become bored with this test since the activities are changed frequently. The test is somewhat cumbersome for the psychologist to administer. For that reason, many psychologists prefer the Wechsler scales instead. However, there are instances where it is very helpful to have another highly standardized and reliable tool to measure a child's intelligence. The Binet fills this need very well.

McCarthy Scales of Children's Abilities
The McCarthy tests children ages 2˝ to 8˝. The purpose of the test is to evaluate the general intelligence level of children. It also identifies strengths and weaknesses in several ability areas. These areas include: verbal, perceptual-performance, quantitative, memory, motor, and general cognitive skills.