The present study also had several limitations. First, the use of changing measures in externalizing problems may limit our ability to draw conclusive inferences regarding developmental change. POM scores across different measures may not be comparable if their items have different severity. For instance, if items on one scale reflect a more severe level of psychopathology (e.g., uses drugs and sets fires) than items on another scale (e.g., argues and brags), the proportion scores may not be equivalent across the two scales. This may not be as much of an issue for the externalizing problem questionnaires in the present study because their items overlap substantially. For other research using more disparate scales, item response theory models or other advanced measurement models may be necessary for calculating trait or scale scores by linking items (e.g., Curran et al., 2008). Even the 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
