Cross-sectional and longitudinal surveys

In addition to the size and distribution of the population surveyed (e.g. national or local), surveys can also be either cross-sectional or longitudinal.

Cross-sectional surveys are used to gather information on a population at a single point in time, for example, on a certain date, or during a particular month. An example of a cross-sectional survey would be a questionnaire that collects data in a single month on how parents feel about adolescent reproductive health services. Alternatively, a questionnaire might try to determine the relationship between two factors at a particular point in time, such as the religious views of parents and whether they accept or reject family planning services.

Longitudinal surveys gather data over a period of time, which may be several months or even several years. The researcher may then analyse changes in the population's demographic or epidemiological features as time passes, and attempt to describe and/or explain these changes. For example, a longitudinal survey could discover that the birth rate in a particular region was falling steadily over time, and that this seemed to be related to a rise in the acceptance of family planning methods in the community.

A community survey may be cross-sectional (e.g. you will conduct a community survey to collect data for your community profile when you are first deployed to a community), but if you continue to collect data on the same topics every year for several years, your survey will become longitudinal.

Usually several people are trained to collect the data for a survey, because a large number of individuals have to be asked the same questions and this would take a long time if one person tries to do it alone. The common survey data-collection tools are questionnaires, interviews and focus groups.