ABSTRACT
Any estimate for a measure of occurrence obtained in an empirical study needs a definition of the study population, from which the observations are obtained. Membership in the population can be defined by residence, occupation, age, sex, or other characteristics, but it always needs a temporal frame. Hence, the study base is composed of a defined segment of population time. The terminology for the population concepts in epidemiology
CONTENTS
Introduction .....................................................................................................................................9 Cohort Studies ............................................................................................................................... 11 Case-Control Studies .................................................................................................................... 14 Other Study Formats .................................................................................................................... 16 Acknowledgments ........................................................................................................................ 18 References ....................................................................................................................................... 19
is far from consistent and well established, but the broad underlying population formed by particularistic bounds (typically space and time, such as geographical region and calendar period) can be called the source population. Within the source population, a researcher can define the intended group for the study, that is, the target population. The captured individuals, or subjects for whom information is available due to, for example, participation, can be called the study population (Figure 2.1). In general, the closer the target population is to the source population in terms of features relevant for the study (exposure distribution and disease risk), the better the generalizability of the results. Correspondingly, a close match in terms of the key features between the target and the study population means that there is less opportunity for selection bias. Yet, the conclusions are made at a more generalized, conceptual level with the results extended as universally applicable to all corresponding circumstances, for example, from observations derived from a sample of Finns to all of humankind as fundamentally similar biological beings.
