Panel data—also known as longitudinal data—tracks the same cross-sectional units (such as individuals, firms, countries, or schools) repeatedly over multiple time periods. By combining the spatial dimension of cross-sectional data with the temporal dimension of time-series data, panel data allows researchers to control for unobserved individual-level heterogeneity, analyze dynamic adjustments, and improve statistical efficiency.
: If your panel identifier is a string variable (e.g., country names), you must first convert it to a numeric variable using the encode command before using xtset . stata panel data
xtivreg y x1 x2 (x3 = z1 z2), fe