same items could have different severities for different informants (e.g., self versus mother), however, so the importance of ensuring that measures are conceptually and empirically equivalent is not specific to studies using changing measures. In any case, keeping the measures identical over time would not resolve the issue of measuring developmental change, because a static measure would likely not have construct validity invariance across the time frame in the present study due to the heterotypic continuity of externalizing problems. Changes in constructs over time require changes in measurement (Eddy et al., 1998); failure to accommodate changes in the form of externalizing problems over time may make differences across age meaningless. Achenbach (2005) emphasized the need for measures to reflect the changing nature of externalizing problems, forming the theoretical foundation for the changes in items across development in the Achenbach scales according to developmentally-relevant forms of externalizing behavior. Because of the developmental relevance of the scales, Owens and Shaw (2003) also modeled externalizing trajectories with different Achenbach scales over time. In any case, we have attempted to show that the trajectories are meaningful insofar as they map onto other important externalizing problems. Moreover, we have shown evidence for the construct validity
