Ecological studies and cardiovascular outcomes research.

Is it better to have a heart attack in the United States or Canada? Ecological studies are epidemiological evaluations in which the unit of analysis is populations, or groups of people, rather than individuals.1 They can answer important questions such as the question posed above that cannot easily be answered using any other form of study design. Ecological studies are conducted by researchers in a variety of disciplines, including outcomes researchers, to study potential causal associations between 1 or more exposure and outcome variables. They are frequently used where alternative study designs are not possible (eg, randomized control trials), such as when investigating the effect of geographical and temporal factors on disease incidence or the effect of a government policy change on health outcomes. In this overview, we provide an introduction to the use of ecological studies in cardiovascular outcomes research, drawing on published examples from our group and others. It is important for clinicians to be aware of both the potential strengths and weaknesses of this type of study design. Because ecological studies are ubiquitous in the medical literature, it is not possible to review all published cardiovascular ecological studies in a systematic manner. Individual-level variables are properties of each person whereas ecological variables are properties of groups, organizations, or places.1,2 All ecological studies are potentially prone to the so-called “ecological fallacy,” which is a term for the biases that may occur when the association that exists between variables at an aggregate level may not represent the true association that exists at an individual level.3 On the other hand, the ecological fallacy may be offset by avoidance of the “individualistic fallacy” whereby one assumes that individual patients are unaffected by the neighborhood in which they live or the setting in which they are treated.4 Complete ecological studies involve studies where all of the variables are ecological in nature, whereas partial ecological studies involve a mixture of individual-level and ecological variables.1 Some epidemiologists argue that ecological studies should be used purely as hypothesis-generating exercises that require further confirmation through other epidemiological study designs such as randomized clinical trials or cohort studies using only individual-level variables.2 An opposing view holds that although such cautions are appropriate, it may never be possible to conduct individual-level studies to answer certain questions and that if strong and biologically plausible effects are observed in an ecological study, the causal inferences may be sufficiently strong to warrant appropriate policy or clinical practice changes.2 To assist readers in evaluating ecological outcomes research studies, we propose a series of questions that can be asked to evaluate their quality, as shown in Table 1. The criteria proposed in Table 1 are not necessarily exclusive to ecological studies but also may apply to other observational studies. Excellent ecological studies do not necessarily have to satisfy all the identified criteria, and readers should exercise their judgment when reviewing these types of studies, in terms of determining what the investigators could have practically done in terms of answering a given research question. In the present article, we discuss several examples of ecological outcomes research studies that have been previously published by our group and others and address both the insights that were gained from these studies and their limitations in light of the methodological issues raised in Table 1.