What Are Longitudinal Data?
Longitudinal data, sometimes referred to as panel data, track the same sample at different points in time. The sample can consist of individuals, households, establishments, and so on. In contrast, repeated cross-sectional data, which also provides long-term data, gives the same survey to different samples over time.
Longitudinal data have a number of advantages over repeated cross-sectional data. Longitudinal data allow for the measurement of within-sample change over time, enable the measurement of the duration of events, and record the timing of various events. For example, suppose the unemployment rate remained high for a long period of time. One can use longitudinal data to see if the same group of individuals stays unemployed over the entire period or if different groups of individuals move in and out of unemployment over the time period.
The NLS surveys gather detailed longitudinal information about the lives of six groups of men and women over time. Each survey group (cohort) consists of 5,000 or more original members. Each cohort was selected to be representative of all people living in the United States at the time of the initial interview and born during a given period.