Ryan MM, et al. - This investigation was undertaken to determine if study partners, compared with participants themselves, provide added information about participant cognition in preclinical Alzheimer disease (AD) trials. The authors tested the assumption that study partners provide meaningful information related to participant cognition cross-sectionally and longitudinally, and evaluated whether amyloid status modified observed effects. They evaluated participant and study partner Everyday Cognition scores and participant Alzheimer Disease Assessment Scale 13-item cognitive subscale information from 335 cognitively normal participant-partner dyads in the AD Neuroimaging Initiative. Although baseline participant reports reasonably anticipate subsequent cognitive change, informants perform better at cross-sectionally recognizing cognitive status as observation time increases. The requirement of the study partner may be necessary for ensuring the validity of the test data, particularly in longer trials.