Intellectual functioning is measured by individual intelligence testing. Care is taken that tests administered are comprehensive, culturally appropriate, and psychometrically sound. Standardized testing is used as part of the diagnostic process. This may include a full-scale IQ test, though it is no longer a requirement for adequate diagnosis. A full-scale IQ score of 70 to 75 indicates a significant deficit, and an IQ below 70 is considered a marker for mental retardation. This being said, one’s score must be interpreted in context. Full-scale IQ scores may not accurately reflect one’s overall intellectual functioning. Thus, broad testing and clinical judgment are necessary to properly assess an individual.
When observing one for intellectual disability, three areas of adaptive functioning are considered: conceptual, social, and practical. Conceptual functioning encompasses language, reading, writing, math, reasoning, knowledge, and memory. Social functioning includes empathy, social judgment, communication skills, the ability to follow rules and to make and keep friendships. Finally, practical functioning refers to independence in areas like personal care, job responsibilities, managing money, recreation, and organizing school and work tasks.
In addition to standardized testing measures, adaptive functioning is assessed through interviews with the individual and others in their life (family members, teachers, and caregivers.) Through this process, the severity of the intellectual disability is discovered and identified as mild, moderate, or severe. Most individuals with intellectual disabilities are categorized as mild.
Symptoms of intellectual disability are usually evident early on. Language or motor skill delays may be seen by age two. This being said, mild levels of intellectual disability may not be apparent until a child enters school and is faced with academic challenges.