Single point in time
The population of a region at a single point in time is defined as the total number of people in that region at that point in time. In some cases, the definition may be restricted to the number of people who are residing long-term in that region.
Interval of time
The population of a region in an interval of time may vary within that interval. There are three competing conventions that are used:
- Standard convention: Mid-interval value: The population is measured at the midpoint of the time interval.
- Beginning-of-interval value: The population is measured at the beginning of the time interval.
- End-of-interval value: The population is measured at the end of the time interval.
Unless otherwise mentioned, we will, for conceptual purposes, use the mid-interval value.
- For regions that have a (preferrably electronically maintained) population register, they can calculate the population at any given point in time.
- For regions that do not have a population register, a census is used. The census is done over a time period rather than at a single point in time, but it can nonetheless be used to reasonably estimate the population value at a single point in time, unless there is significant day-to-day fluctuation in the population.