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Selection of Normative Group Affects Rates of Mild Cognitive Impairment in Parkinson's Disease
Kathryn A Wyman-Chick1,2,3, Phillip K Martin4, Daniel Weintraub5,6
1HealthPartners Neuroscience Center, Saint Paul, MN.
Objective:
The objective of this study was to examine the impact of different methods of standardizing cognitive data in the Parkinson's Progression Marker Initiative.
Methods:
Cognitive data from 423 participants with Parkinson's disease were included (age = 61.7 [9.7], education = 15.6 [3.0]). Internal norms were calculated using the group mean and standard deviation of the healthy control group. Published norms were compared to the overall group mean of and to age-stratified norms from healthy controls for each neuropsychological test over 4 visits. Rates of mild cognitive impairment were calculated using established criteria.
Results:
The use of internal norms resulted in lower standardized scores than published norms on all tests with the exception of memory and processing speed (P ≤ .001). Individuals were 1.5 to 2.1 times more likely to be diagnosed with mild cognitive impairment using internal norms than published norms.
Conclusions:
Standardization approaches with cognitive data are not interchangeable. Selection of a normative comparison group impacts research and clinical interpretations of cognitive data. © 2018 International Parkinson and Movement Disorder Society.