SEPLINE: Socioeconomic Position in Epidemiological Research-A National Guideline on Danish Registry Data.

Background: Socioeconomic differences in health have become an increasing public health concern and priority, leading to a growing number of studies investigating the relationship between socioeconomic position and health outcomes. However, variability in methodological practices hampers the comparability of findings and leads to inefficiencies, as researchers invest substantial resources in selecting appropriate variables and methods. To address these challenges, the SEPLINE initiative was established to develop a methodological guideline aimed at enhancing the comparability, quality, and feasibility of socioeconomic research using Danish registry data. Methods: The guideline was developed through a consensus-driven approach involving an interdisciplinary group of stakeholders from Danish universities, research institutions, and data warehouses. The guideline addresses socioeconomic position as an exposure based on data from Danish registries, with the cancer continuum applied as a case outcome to illustrate its application. The development process included two collaborative workshops informed by a pre-workshop questionnaire. Workshop I (spring 2024) focused on socioeconomic indicators, data collection, and data management, featuring expert presentations and group discussions. Workshop II (fall 2024) addressed analytical methods, including causal inference challenges and income/wealth assessment methods. Insights from these workshops were integrated into iterative refinements of the guideline. Conclusions and Implications: The guideline provides a structured framework for conducting socioeconomic epidemiological research using Danish registry data, offering specific information on data sources and recommendations about variable selection, measurement timing, and data handling. While tailored to Danish registry-based cancer research, the guideline's methodological principles have broader applicability to other diseases and international contexts. By emphasizing transparency, theoretical grounding, and methodological rigor, SEPLINE aims to advance the study of social determinants of health. Researchers are encouraged to use the guideline as a relevant starting point and adapt it to their specific study populations and research questions, ensuring its relevance across diverse settings.