Fig. 1.

Hierarchical structure of a dataset. Assume that in a neuroimaging study a group of S subjects are recruited to perform a task (e.g., the Eriksen Flanker task; Eriksen and Eriksen, 1974) with two conditions (e.g., congruent and incongruent) and each condition is instantiated with T trials. The collected data are structured across a hierarchical layout of four levels (population, subject, condition and trial) with total 2 × S × T = 2ST data points at the trial level compared to S across-condition contrasts at the subject level.