Base rates of low neuropsychological test scores in older adults with subjective cognitive impairment: Findings from a tertiary memory clinic.

Base rates of low scores are typically determined from normative samples, which differ from clinical samples. We examined base rates of spuriously low scores for 93 older adults with subjective cognitive impairment presenting to a memory clinic. Crawford's Monte Carlo simulation algorithm was used to estimate multivariate base rates by calculating the percentage of cognitively intact memory clinic patients who produced normed scores at or below the 5th percentile. Neuropsychological tests included: Weschler Adult Intelligence Scale block design, digit span backwards, coding, Weschler Memory Scale logical memory immediate/delayed; California Verbal Learning Test immediate/delayed memory; Brief Visuospatial Memory Test immediate/delayed; and Delis-Kaplan Executive Functioning category switching, letter number sequencing, and inhibition/switching. An estimated 33.58% of the cognitively intact memory clinic population would have one or more low scores, 14.7% two or more, 6.55% three or more, 2.94% four or more, and 1.31% 5 or more due to chance. Base rates were then applied to a subset of clinical data: all with dementia and most with MCI had low scores that exceeded the base rates. Determining base rates of spuriously low scores on a neuropsychological battery in clinical samples could reduce false positives by using empirical adjustments for expected low scores.