Data collected over several time periods on a number of individual units. A panel data set is balanced if the same time series observations are available for all cross-sectional units; otherwise it is said to be unbalanced. Commonly used methods of analysis of panel data include pooled least squares, fixed effects, and random effects. Not every data set that combines time series and cross-sectional observations represents a panel; for example, a household survey where different households were surveyed in different waves is not, strictly speaking, a panel.