The four levels of severity are defined on the basis of adaptive functioning in three primary domains: conceptual, social, and practical.
Disability is described as mild, moderate, severe, or profound; however, these designations are based primarily on ability and needed supports rather than on IQ scores.
Mild ID: constitutes the largest group; estimated that 85% of people with ID have the mild form of the disorder. Children often show small delays in development during the preschool years, but typically are not identified until academic or behaviour problems emerge during the early elementary years. During adulthood, they usually achieve social and vocational skills adequate for minimum self-support, but may need supervision, guidance, and assistance.
Moderate ID: constitute about 10% of individuals with ID. They are usually identified during the preschool years, when they show delays in reaching early developmental milestones. Entering school, they may communicate through a combination of single words and gestures, and show self-care and motor skills similar to an average 2-3 year old. Often require supportive services to function on a daily basis. They benefit from vocational training and, with moderate supervision, can attend to personal care. By adulthood, typically adapt well to living in the community and can perform semiskilled work under supervision.
Severe ID: constitue about 4% of persons with ID. Most suffer one or more organic causes of impairment, such as genetic defects, and are identified early because of substantial delays. They will require some special assistance throughout their lives. Acquire little or no communicative speech. Most adapt well to life in the community, in group homes or with their families.
Profound ID: constitute about 2% of those with ID. Typically identified in infancy because of marked delays in development. They show considerable impairments in sensorimotor functioning. They are able to learn only rudiments of communication skills, and they require intensive training to learn basic eating, grooming, toileting, and dressing behaviours. They require lifelong care and assistance. Almost all of these individuals show organic causes for their ID, and many have severe co-occurring medical conditions. Most live in supervised group homes or small, specialised facilities.