"Measurement Invariance in Longitudinal Studies of Trauma and Psychological Distress"

Demonstrating the integrity of a construct is recognized as a prerequisite to the quantification of change in longitudinal studies. Methods for evaluating this measurement invariance are available in the context of structural equation models. These methods allow some flexibility when some indicators of the construct differ across time. Shifting indicators may be common in trauma research, where disruptions are typical concomitants of a traumatic event such as community violence or a natural disaster. We illustrate a two-step approach to assessing measurement invariance prior to evaluating change that requires a single scale to remain constant across time. At the first step, the common scale is evaluated for invariance at the item level. At the second step, the common scale is used to set the metric and the construct’s factorial invariance is assessed. Using data from a study of 151 Kuwaiti children exposed to the Gulf war, assessed in 1993 and again in 2003, we illustrate the approach with a measure of posttraumatic stress symptoms using Davidson’s self-rating scale as the common scale over time. Fourteen out of 17 items were shown to have invariant loadings on a 3-factor confirmatory analysis, although items tended to have greater reliability when participants were older. Scales to measure depression and anxiety were also administered at both times, but different scales were used at each time. In 1993, depression was assessed with the Children’s Depression Inventory and anxiety was assessed with the Revised Children’s Manifest Anxiety Scale. In 2003, depression was assessed with the Beck Depression Inventory and anxiety was assessed with the Spielberger’s Trait Anxiety scale. Using the three latent variables from step 1 and the two additional measures of depression and anxiety, a second-order factor of psychological distress was specified at each time. Invariance in the loadings of the common scales was established, allowing the loadings of the different scales to be estimated freely at each time. The means of the second-order latent variable may now be compared over time. The presentation of the two-step approach will be embedded in a broader presentation that will cover an introduction to measurement invariance, how invariance may be assessed in the context of structural equation models, latent models for dealing with shifting indicators over time, and other methodological issues associated with measurement invariance in the context of a longitudinal study.