What counts as knowing? Reflections on evidence hierarchies in qualitative longitudinal and participatory research...

What counts as evidence remains contested at the research-policy interface, with consequences for qualitative researchers seeking to demonstrate impact. Despite increasing recognition of lived experience and participatory inquiry, entrenched evidence hierarchies continue to privilege quantification, scale and standardisation as markers of methodological rigour and policy relevance. These dynamics pose particular challenges for research with marginalised groups whose lives remain under-specified within administrative data systems.

Drawing on insights from a qualitative longitudinal and participatory research programme with young fathers, this article argues that evidential credibility tied to prevalence and sample size systematically constrains the visibility, uptake and impact of qualitative research. Contesting these hierarchies requires both critical attention to how they operate and a broader understanding of what constitutes legitimate impact.

Two encounters demonstrate how evidence hierarchies are reproduced through everyday institutional practices that privilege quantification as a proxy for credibility and policy relevance. The recurring demand to establish how many young fathers there are nationally exposes how populations under-specified within administrative and frontline data systems struggle to achieve evidential visibility. Meanwhile, a government quality assurance process privileging sample sizes exceeding 1,000 participants illustrates how scale operates as a marker of legitimacy despite the distinctive depth, relationality and temporal insight generated through qualitative longitudinal research. Together, these dynamics create a self-reinforcing cycle in which already under-researched populations remain empirically and institutionally marginalised.

Funders, policy makers, practitioners and academic institutions must support processual, relational and longitudinal models of impact that recognise qualitative inquiry as central to credible evidence.