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The term population-based is traditionally used to describe a study that involved a defined “general population”, as opposed to hospital-based or occupation-based populations. Epidemiologic studies have a tacit need to be based in populations, and as such, most epidemiologic studies can be loosely considered as population-based. The etymology of the word epidemiology arises from epi-demos, meaning upon the people. By definition, epidemiology has as one of its fundamental concepts, the population. But conflicting definitions in epidemiology are common where the names of study designs and concepts are concerned. Readers of epidemiologic literature should be aware that several terms are used idiosyncratically by epidemiologists. The first section below describes population-based designs, and following this is a short section on the concept of population. Population-based designs Traditionally, epidemiologic studies were often labeled by various methodological descriptors, indicating the origins of the source population being exploited for the study. For example, hospital-based studies and industry-based studies obviously try to accrue patients and workers, respectively. But these are not universally referred to as population-based approaches. The common usage of the term implies sampling of individuals from the general population, one that is defined by geopolitical borders. Sometimes even case-control studies (another epidemiologic design) are not considered population-based because the sampling is often perceived as outcome-based. This, however, is at odds with other usage. The use of the term population-based is a misnomer. What is actually being referred to is a study that uses a directly defined population (as opposed to indirectly defined). For example, a case-control study can begin by defining a source population and then directly sampling from that population to form the control series. Take, as one example, this text concerning a case-control study in Montreal (Parent et al. 2006): The present paper describes associations between diesel and gasoline engine emissions and lung cancer, as evidenced in a 1979-1985 population-based case-control study in Montreal, Canada. Cases were 857 male lung cancer patients. Controls were 533 population controls and 1,349 patients with other cancer types. This study actually serves to illustrate a more modern explanation of the meaning of ‘population-based’ and how epidemiologic studies have the tacit reliance on a population. This Montreal study, in theory, provides two equivalent samples of the source population (metropolitan Montreal residents during 1979 to 1985). One, a directly defined population, was formed (533 controls) by sampling from electoral lists of Montreal. The authors refer to this series as population controls. Different lists were used over the course of the study, relating to the dynamic population membership of those moving into or out of Montreal over the years. But there was an alternative approach used in the Montreal study that secondarily defined its population. That is, the source population was defined indirectly via the method of identifying the cases of lung cancer (diagnoses in pathology departments of various hospitals). The second control group (the 1349 “cancer controls”) were identified as men with other cancer diagnoses, found in the same hospitals. In this fashion, the source population is actually being defined as the catchment populations of the hospitals, but since the Montreal study restricted itself to Montreal residents, both “population controls” and “cancer controls” are (in theory) representative of the same Montreal source population. Populations References: Further reading: |
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