Revival of ecological studies during the COVID-19 pandemic

Excerpt

The COVID-19 pandemic has brought ecological studies to the fore. In ecological studies, the units of analysis are groups rather than individuals, usually because available data are on the group-level. The purpose of an ecological study can be to learn about exposure effects on individual-level risks or to learn about effects on the group- or population-level. Keeping this distinction in mind is essential to both the researcher and to the reader of a study, but conflation is common. Ecological studies are indispensable to follow time trends and distributions of disease in populations and the surge of these studies during the pandemic is a testament to this. A commentary in last year’s December issue of EJE also stressed that the essence of epidemiology still is comparison of disease rates in cleverly chosen populations [1]. Comparisons in ecological studies generally only provide weak, if any, evidence for causality on the individual level. They are notorious for the so-called ecological fallacy, which may occur when one assumes that relations on the population level also hold on the individual level. However, the scientific quality may vary considerably depending on the study design, setting and choice of comparison groups. The added value of a study also depends on the current state of knowledge. Ecological studies have provided invaluable insights and helped to advance etiological hypotheses. The Seven Countries Study, started by Ancel Keys in the 1950’s, led to novel understanding of the relation between dietary fat and coronary heart disease [2]. Research by McMahon and others on the relation between reproductive history, lactation, and breast cancer risk is a classic example of how hypotheses originating from ecological associations were subsequently tested in individual-level studies [3]. The INTERSALT study is yet another example, where a key strength was that both within-population and cross-population analyses regarding salt intake and blood pressure were undertaken [4]. …