Panel Data: a group of entities observed over time.
A panel data set is a combination of cross-section and time series. It is a cross-section of n entities observed over t periods. It is also referred to as longitudinal data.
Below is a screen shot of a panel data set on average SAT scores (VSAT, MSAT, and WSAT), the percent of eligible students who took the exam, and real per pupil expenditures for public K-12 education by state for the eyars 2008-2011. Notice that the first two columns are identifiers. With a panel data set, we need two identifiers: one that identifies the cross section unit (main_id) and one that identifies the time period (year). Each row of data pertains to a specific state in a specific year.
You can download this .csv file by clicking on this link: SAT_paneldata
Notice that there are no empty rows, simple variable names without spaces, no units for number values (no % signs). When you construct your own data set, follow these practices.