Practice effects due to serial cognitive assessment: Implications for preclinical Alzheimer's disease randomized controlled trials

Abstract

Introduction: Practice effects are characteristic of nearly all standard cognitive tasks when repeated during serial assessments and are frequently important confounders in clinical trials.

Methods: We summarize evidence that gains in neuropsychological test performance scores associated with practice effects occur as artifactual changes associated with serial testing within clinical trials. We identify and emphasize such gains in older, non-cognitively impaired individuals and estimate an effect size of 0.25 for composite cognitive measures in older populations assessed three times in a 6- to 12-month period.

Results: We identified three complementary approaches that can be used to attenuate practice effects: (1) massed practice in a prebaseline period to reduce task familiarity effects; (2) tests designed to reduce practice-related gains so that item-specific driven improvements are minimized by using tasks that minimize strategy and/or maximize interitem interference; and (3) well-matched alternate forms.

Discussion: We have drawn attention to and increased awareness of practice effect-related gains that could result in type 1 or type 2 errors in trials. Successfully managing practice effects will eliminate a large source of error and reduce the likelihood of misinterpretation of clinical trials outcomes.

Keywords: Alzheimer's disease; Clinical trials; Cognition; Neuropsychology; Practice effects; Preclinical Alzheimer's disease; Serial assessment.

Fig. 1

Change from year 0 data in the ADNI studies for the Rey Auditory Verbal Learning Test. Ascending values reflect improvements and descending impairments. The two parallel forms (A and B) are clearly not of equivalent difficulty. Abbreviations: ADNI, Alzheimer's Disease Neuroimaging Initiative; AVLT, Auditory Verbal Learning Test; LS, Least Square.