What Is a Population?
Until now, we have implicitly defined "population" very broadly, generally equating it to the population of a country. Populations however do not need to be as large, or to be confined within administrative boundaries: they simply represent a collection of individuals with a common characteristic. In the case of country, this common characteristic is residence. In public health, we also frequently define populations according to their risk behaviors (e.g., injecting drug users) or their medical history (e.g., cancer survivors). The techniques and tools of demography can also be usefully applied to such populations: for example, individuals may enter the population of injecting drug users in New York City by taking up injecting drug use or by moving from another city; they may exit the population if they quit injecting, are incarcerated or possibly die. The size (and growth) of the IDU population will depend on which flow is quantitatively more important.