A longitudinal examination of the measurement equivalence of mental health assessments in two British birth cohorts

Valid inference from the investigation of mental health relies – among others – on the assumption of no measurement error. However, it is well known that data from self-reported measures are likely to be biased by some process that is driven by the respondent’s personality and/or circumstances. We capitalised on data available in two nationally representative birth cohorts, the National Child Development Study (1958 birth cohort) and the 1970 British Cohort Study to formally test the longitudinal measurement equivalence of the nine-item version of the Malaise Inventory, a measure of psychological distress. The inclusion of identical assessments of mental health in adulthood in both cohorts allowed us to evaluate their measurement properties and investigate whether the passage of time has differentially affected the interpretation of mental health assessments. To do so, we employed methods within the generalised latent variable measurement modelling framework and related extensions for formally testing measurement invariance. We found that the passage of two decades and more in both cohorts have not influenced how participants respond to the short version of the Malaise Inventory. The observed scalar invariance of the short version of the Malaise Inventory implies that potential sources of bias such as age effects, survey design, period effects, or cohort specific effects did not influence the way participants in the two cohorts respond to the symptoms described in the Malaise Inventory. Our results offer some reassurance for the extent to which self-reported mental health survey questions are affected by systematic sources of error.