Abstract
The article focuses on demographic data sources and how they have changed over time. Generally, demographic data is divided into three classes: primary, secondary, and tertiary.
The most important data, which are highly reliable, are primary data. The category contains population censuses, vital statistics, migration statistics , statistics on morbidity, population registers, sample surveys, and historical sources.
Secondary data sources are less reliable and are generally compiled for reasons other than population study. Tertiary data sources include, for instance, popular non-fiction literature and should only be considered with extreme caution.