What is necessary to determine sub-average intellectual functioning?

To identify sub-average intellectual functioning, a critical benchmark is the statistical measurement of IQ scores. Specifically, a determination of sub-average intellectual functioning is indicated when an individual's IQ score falls two or more standard deviations below the mean on standardized intelligence tests.

Standardized tests are designed so that the mean IQ score is set at 100, with the majority of the population scoring within one standard deviation (about 68% fall between 85 and 115). Thus, scores below 70 (which represent two standard deviations below the mean) are often used as a criterion for identifying individuals who may be classified as having intellectual disabilities. This approach is grounded in both statistical norms and the frameworks established by diagnostic criteria for various intellectual and developmental disabilities.

In contrast, scores above the mean or average IQ scores for specific age groups do not provide relevant data for identifying sub-average functioning. Parental evaluations can offer insights into a child's capabilities but do not constitute standardized measures necessary for diagnosis. Hence, the most reliable method for determining sub-average intellectual functioning relies on quantifiable data derived from standardized testing, specifically looking for scores that definitively place an individual two standard deviations below the mean.