Keywords: Longitudinal, Hard-to-reach, Birth cohort, Response rates, Experiments, United Kingdom

Longitudinal birth cohort studies are vital for understanding the development and outcomes of successive generations of children, though there is increasing recognition that often those families who are of most interest from a research and policy perspective are less likely to be recruited and retained in national studies. Therefore, designing an inclusive survey that can effectively reach hard-to-reach populations is a key challenge, where retention of those groups is critical for well-powered analyses and subsequent policy impact.
The UK Early Life Cohort Feasibility Study (ELC-FS) tested the feasibility of conducting a new UK-wide birth cohort study, by collecting information about several thousand babies aged between 8-12 months old, and their families in 2023-2024. It is funded by Economic and Social Research Council and led by the Centre for Longitudinal Studies at University College London with interviewing carried out by Ipsos. Evidence from the feasibility study will be used to inform the potential commissioning of a much larger main study which will be the first UK-wide birth cohort study in 20 years. The ELC-FS has a very strong focus on inclusivity and was designed to maximise representation of ‘less often heard’ groups. It includes sample boosts of babies born into disadvantaged and ethnic minority families, as well as Scotland, Wales and Northern Ireland, and direct recruitment of fathers including those living in their own households.

The study involved 30-60 minute interviews with mothers and fathers about children’s social and economic environments, their health, well-being and development. We also collected saliva and oral fluid swabs for DNA extraction with a randomised subgroup, and record linkage consents where we experimentally tested different consent wording. Interviews were carried out primarily face-to-face, with web, phone and video interviewing also used. We also included experiments to test the effectiveness of targeted differential incentives and conditional incentives.

This paper will give an overview of the inclusive design and response maximisation efforts in the ELC-FS. We will also present some early findings relating to representativity, response rates, and the experimental components of the feasibility study: we achieved a high proportion of lower-income families, response rates among ethnic minority groups tended to be higher than others, and we found differences in consent rates to data linkages and saliva provision among lower income and ethnic minority groups families.