Many of my students got confusion regarding this terminology used by gretl for importing data. Stacked time series and Stacked cross sections. Can you provide a brief description about the organization of the data like below or any other suitable name or example? The help is ok, but these options are easy to understand. Why grouped by? This is a very famous verb in dplyr R package. Stacked time series (grouped by Units) Stacked cross sections (grouped by Time)