Example identification of scan groups at the four levels of variation. The MASiVar data set consists of scans across multiple sessions, scanners, and subjects that can be grouped in order to satisfy intrasession, intersession, interscanner, and intersubject criteria. The scans in each of these groups should produce the same measurements; thus, quantification of variation within groups provides an estimate of variability. For the intersession, interscanner, and intersubject groups, scans are randomly shuffled within sessions before grouping.