The development of sampling theory and techniques in the early part of the twentieth century led to the sample survey becoming one of the most widely used methods of data collection in social research. Interview surveys based on a sample of the members of an organisation, occupation or local community often constitute one source of information for case studies, and sample surveys are frequently used as the main element of longitudinal studies, or of experimental research designs. Sampling alone as a research technique has even wider usage; for example, non-random sampling may be used to select people for qualitative research, and sampling techniques are often used in studies based on administrative records. However, sample surveys can constitute a research design in their own right, and it is in this capacity that we consider them here.