On Fri, 11 Feb 2011, Summers, Peter wrote: I believe gretl does this correctly, as per the user's instructions. (I have constructed and manipulated some dummy panel datasets to verify this.) But you will have to pay very careful attention to your specification of how many cross-sectional units and how many time periods compose the panel (bearing in mind what you're claiming to be the current organization). You can flip easily back and forth between the two structures only if the numbers of units and periods are the same. Otherwise it is easy to scramble the dataset much like a Rubik's cube. A more foolproof method is to ensure that the dataset contains two variables that serve as indices of the unit and the period, and to employ the "Use index variables" method in the panel structure dialog. (Stata, for instance, only accepts this method for defining a panel.) Allin Cottrell