We then used Item Response Theory to derive externalizing scale scores that optimize available data and are sensitive to item differences in severity, behavioral repertoire, and development (for details of these analyses, see Hussong, Wirth et al., 2007). Item Response Theory has several advantages over traditional proportion scores (see Curran et al., 2007; Curran et al., 2008) including differential weighting of individual items (allowing for scale scores that account for the severity of the specific externalizing symptoms) and tests of differential item functioning that identify the extent to which items vary in their relation to externalizing symptomatology over sub-populations (such as that defined by study membership).
