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In statistics and econometrics, the term **panel data** refers to multi-dimensional data frequently involving measurements over time. Panel data contain observations on multiple phenomena observed over multiple time periods for the same firms or individuals. In biostatistics, the term **longitudinal data** is often used instead,^{[1]}^{[2]} wherein a subject or cluster constitutes a panel member or individual in a longitudinal study.

Time series and cross-sectional data are special cases of panel data that are in one dimension only (one panel member or individual for the former, one time point for the latter).

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balanced panel: | unbalanced panel: | |
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In the example above, two data sets with a two-dimensional panel structure are shown, although the second data set might be a three-dimensional structure since it has three people. Individual characteristics (income, age, sex. educ) are collected for different persons and different years. In the left data set two persons (1, 2) are observed over three years (2003, 2004, 2005). Because *each* person is observed *every* year, the left-hand data set is called a **balanced panel**, whereas the data set on the right hand is called an **unbalanced panel**, since Person 1 is not observed in year 2005 and Person 3 is not observed in 2003 or 2005.

A panel has the form

where is the individual dimension and is the time dimension. A general panel data regression model is written as Different assumptions can be made on the precise structure of this general model. Two important models are the fixed effects model and the random effects model. The fixed effects model is denoted as

are individual-specific, time-invariant effects (for example in a panel of countries this could include geography, climate etc.) and because we assume they are fixed over time, this is called the fixed-effects model. The random effects model assumes in addition that

and

that is, the two error components are independent from each other.

*German*Socio-Economic Panel (SOEP)- Household, Income and Labour Dynamics in Australia Survey (HILDA)
- British Household Panel Survey (BHPS)
- Survey of Family Income and Employment (SoFIE)
- Survey of Income and Program Participation (SIPP)
- Lifelong Labour Market Database (LLMDB)
- Panel Study of Income Dynamics (PSID)
- Korean Labor and Income Panel Study (KLIPS)
- Chinese Family Panel Studies (CFPS)
- German Family Panel (pairfam)
- National Longitudinal Surveys (NLSY)

Main article: Multidimensional panel data

Baltagi, Badi H., 2008. __Econometric Analysis of Panel Data__, John Wiley & Sons, Ltd.

Hsiao, Cheng, 2003. __Analysis of Panel Data__, Cambridge University Press.

Davies, A. and Lahiri, K., 2000. "Re-examining the Rational Expectations Hypothesis Using Panel Data on Multi-Period Forecasts," __Analysis of Panels and Limited Dependent Variable Models__, Cambridge University Press.

Davies, A. and Lahiri, K., 1995. "A New Framework for Testing Rationality and Measuring Aggregate Shocks Using Panel Data," Journal of Econometrics 68: 205-227.

Frees, E., 2004. __Longitudinal and Panel Data__, Cambridge University Press.