Panel data refer to longitudinal data sets generated by observing a sample of individuals over time. Therefore, a panel has two dimensions involving individuals and time. Panels are a mix of cross sectional information and time series information.
1. The National Longitudinal Survey of Labor Market Experience follows groups of individuals that participate in the labor force, through regular annual interviews.
2. The Panel Study of Income Dynamics (PSID) collects annual economic information on a national sample of about 6,000 families and 15,000 individuals since 1968.
Hsiao, C. (1986): Analysis of Panel Data. Cambridge University Press.