EViews POOL objects operate on variables that have special two-part names. The first part is the name of the variable, and the second part of the name is the cross-section identifier that indicates which cross-sectional unit the variable belongs to. It is good practice (though not necessary) to begin cross-section identifiers with an underscore mark to make the full variable names more readable. Example: Suppose we want to work with a panel data set on the USA, Canada, and Mexico. The variables that we want to use are GDP, Population, and Trade Flows.