3. The longitudinal study designLongitudinal research is used when continuous collection of factual information is required.In the longitudinal study, the research population will visit regularly, usually for a long time, to collect the required information.These intervals are not fixed, so their length varies from study to study. The interval may be as short as one week or as long as one year.Regardless of the size of the interval, the type of information collected each time is the same.Although the data collected are from the same study population, they may or may not come from the same interviewee.A longitudinal study can be regarded as a series of repeated cross-sectional studies.