The essential feature of a longitudinal survey design is that it provides repeated observations on a set of variables for the same sample units over time. The different types of longitudinal studies (e.g. retrospective studies, panel surveys, and record linkages) are distinguished by the different ways of deriving these repeated observations. In a panel survey, repeated observations are derived by following a sample of persons (a panel) over time and by collecting data from a sequence of interviews (or waves). These interviews are conducted at usually fixed occasions that in most cases are regularly spaced.

There are many variations under this general description of a panel survey, including (a) cohort panel surveys, (b) household panel surveys, and (c) rotating panel surveys. These three types of panel ...