In demography it is customary to make a distinction between cross–sectional and longitudinal approaches. Cross-sectional data provide a picture of a population, or population segment, with regard to specific demographic characteristics at a certain moment or during a limited period (usually a year) in time. Longitudinal data can only be obtained when a population or population segment is followed through time, which requires access to a continuous flow of data. Collecting cross-sectional data is like taking stock, hi a well-known introduction to demography, the subject of fertility, for instance, is treated both ways. There is a part on period fertility which discusses cross-sectional fertility data and their methods of analysis, and there is a part on cohort fertility, which discusses fertility patterns which evolve over time and their methods of analysis (Newell, 1988).