Base rates of "impaired" neuropsychological test performance among healthy older adults.

Base rates of deficient neuropsychological test performance were evaluated among 132 neurologically healthy older normal adults using a variety of measures commonly employed in the "flexible-battery"approach to neuropsychological assessment. Subjects were divided into three age groups (50 to 59, 60 to 69, and 70 to 79 years). Despite the healthy status of our sample, most tests yielded at least some proportion of subjects earning scores in the borderline and impaired ranges (1.3 and 2.0 standard deviations below the age-group mean, respectively). Across the battery of measures, 73% of subjects earned a borderline score on at least one measure, and 20% of subjects earned at least two scores in the impaired range on separate tests. The proportion of subjects consistently earning borderline or impaired scores across multiple measures within specific cognitive domains was generally lower. Results illustrate the problems in interpreting isolated low scores, and the need to consider false-positive base rates in drawing inferences from poor test performance.