Mental retardation grading scale can be categorized into four levels according to the degree of mental retardation: mild, moderate, severe and very severe. 1. Mild: IQ between 2.01 and 3 standard deviations. 2. Moderate: IQ between 3.01 and 4 standard deviations. 3. Severe: IQ between 4.01 and 5 standard deviations. 4. Extremely severe: IQ above 5.01 standard deviations. Mental retardation is categorized into three types of conditions according to their causes: one is mental retardation caused during the period of mental development; the second is mental impairment caused by various reasons after the mental development has matured; and the third is dementia caused by the obvious decline of mental ability in old age.