You could do better tomorrow: Nonverbal reasoning speed fluctuates from day-to-day while working memory performance is stable

On some days we feel like we are not performing at our best. However, whether these experiences align with substantive differences in cognitive performance has not been studied systematically. We analyse dense time-series data of children’s performance on nonverbal reasoning (n=449; t=454,913) and visuospatial working memory (n=4150; t=1,048,576) tasks using dynamic structural equation models to describe their pattern of instability across trials and days. Our model comparison confers domain-specific results, with children showing evidence for fluctuations in their nonverbal-reasoning speed from day-to-day, but stability in working-memory performance. Children’s day-to-day fluctuations in nonverbal reasoning speed were 1.1 the size of individual differences in mean response-speed. We show that day-to-day fluctuations in cognitive performance are more than folk intuition, argue that their neglect is problematic for translational and epistemic reasons, and demonstrate how a better understanding of cognitive performance as a dynamic phenomenon can improve cognitive assessment and theory construction.