More than how many standard deviations below the mean is typically considered impairment?

• A. One
• B. Two
• C. More than two
• D. Three

Answer: (C)

Impairment is defined in this context by how far a person’s score deviates from the average. When we express deviation in standard deviation units, a score that is more than two standard deviations below the mean falls into a range that is well outside typical variation and is used to flag clinically meaningful deficits. This corresponds to roughly the lowest 2.5% of scores on a normal distribution, which is why this level is considered impairment. Scores exactly two below sit at the boundary, while one below is still within ordinary variability, and three below would indicate an even more severe deficit. So, more than two standard deviations below the mean best captures the commonly used impairment threshold.