A key notion that has received much attention in epidemiology over the past few years has been that not all disease determinants can be conceptualized as individual-level attributes, hence the need to consider features of the groups to which individuals belong when studying the causes of ill health. This has led epidemiologists and public health researchers to rethink the ideas on ecologic studies and ecologic variables traditionally espoused in epidemiology (1–6). This reconceptualization of ecologic or group-level variables has been manifested, for example, in recent interest and debate on the possible health effects of group-level constructs, such as income inequality (7, 8), social capital (9, 10), and neighborhood characteristics (11–14). In this context, the advent of the statistical technique of multilevel models has been viewed as especially promising because of its ability to incorporate both group-level and individual-level predictors in the study of health (4, 15–17). The idea that factors beyond individuals, referred to as group-level, ecologic, macro-level, or population-level factors (1, 3, 5, 18, 19), are important to health is not new. Two well-known examples include the concept of herd immunity in infectious diseases and Rose’s distinction between the causes of cases and the causes of incidence rates in chronic diseases. Herd immunity implies that a person’s likelihood of contracting an infectious disease depends in part on the level of immunity in the population to which he or she belongs (20). In his seminal paper, “Sick Individuals and Sick Populations,” Geoffrey Rose (18) discusses a related concept: the idea that studies that focus on what distinguishes sick individuals from healthy individuals within a population or group may miss important disease determinants. This is because population-level factors are invariant within a population and, hence, cannot be investigated in studies restricted to comparisons of individuals within a population (21). To detect these factors, researchers need studies that compare different populations (or groups) and investigate population-level (or group-level) factors.
Stored only when both papers are in the corpus — 304 citations exist upstream.