Panel data (or longitudinal data) tracks the same subjects (individuals, firms, countries) over multiple time periods

of one person, but that didn't represent the whole population. Finally, Sam discovered Panel Data

• Overall: Total variation
• Between: Variation across panels (means of each panel)
• Within: Variation over time within panels

Within: Variance within the same entity over time (time-series variation).

xtreg y x, fe vce(cluster id)

Stata Panel Data May 2026

Panel data (or longitudinal data) tracks the same subjects (individuals, firms, countries) over multiple time periods

of one person, but that didn't represent the whole population. Finally, Sam discovered Panel Data

• Overall: Total variation
• Between: Variation across panels (means of each panel)
• Within: Variation over time within panels

Within: Variance within the same entity over time (time-series variation).

xtreg y x, fe vce(cluster id)