Because it is difficult to measure in the population, there is only general agreement on how many people have mental retardation. This translates to about 2.5 million noninstitutionalized people having mental retardation.
Estimates vary by age and definition used.
People, age 15 and over, reporting mental retardation as a cause of disability in the SIPP in 1992 totaled 501,000. The number of people with mental retardation causing an activity limitation goes down with age.