Integrating and extending cohort studies: lessons from the eXtending Treatments, Education and Networks in Depression (xTEND) study

Background: Epidemiologic studies often struggle to adequately represent populations and outcomes of interest. Differences in methodology, data analysis and research questions often mean that reviews and synthesis of the existing literature have significant limitations. The current paper details our experiences in combining individual participant data from two existing cohort studies to address questions about the influence of social factors on health outcomes within a representative sample of urban to remote areas of Australia. The eXtending Treatments, Education and Networks in Depression study involved pooling individual participant data from the Australian Rural Mental Health Study (T-0 N = 2639) and the Hunter Community Study (T0 N = 3253) as well as conducting a common three-year follow-up phase (T-1 N = 3513). Pooling these data extended the capacity of these studies by: enabling research questions of common interest to be addressed; facilitating the harmonization of baseline measures; permitting investigation of a range of psychosocial, physical and contextual factors over time; and contributing to the development and implementation of targeted interventions for persons experiencing depression and alcohol issues. Discussion: The current paper describes the rationale, challenges encountered, and solutions devised by a project aiming to maximise the benefits derived from existing cohort studies. We also highlight opportunities for such individual participant data analyses to assess common assumptions in research synthesis, such as measurement invariance, and opportunities for extending ongoing cohorts by conducting a common follow-up phase. Summary: Pooling individual participant data can be a worthwhile venture, particularly where adequate representation is beyond the scope of existing research, where the effects of interest are small though important, where events are of relatively low frequency or rarely observed, and where issues are of immediate regional or national interest. Benefits such as these can enhance the utility of existing projects and strengthen requests for further research funding.