Qualitative research in epidemiology: theoretical and methodological perspectives.

Increasingly, modern epidemiology has adopted complex causal frameworks incorporating individual- and population-level determinants of health. Despite the growing use of qualitative methodologies in public health research generally, discussion of causal reasoning in epidemiology rarely considers evidence derived from qualitative research. This article argues for a coherent role for qualitative research within epidemiology through analysis of the principles of causal reasoning that underlie current debates about causal inference in epidemiology. It introduces two approaches to causal inference by Russo and Williamson (2009) and Reiss (2012) that emphasize the relevance of both the nature of causation and how knowledge is gained about causation in assessing evidence for a causal relation. Both theories have scope for incorporating multiple types of evidence to assess causal claims. We argue that these theories align with the empirical focus of epidemiology and allow for different types of evidence to evaluate causal claims, including evidence originating from qualitative research; such evidence can contribute to a mechanistic understanding of causal relations and to understanding the effects of context on health-related outcomes. Finally, we discuss this approach in light of previous literature on the role for qualitative research in epidemiology and implications for future epidemiological research.