Hi everybody, this is a bit of an advanced panel data handling problem, but at the same time practically relevant I think... Perhaps it might even qualify as a bug, but let's see: I'm wondering whether appending a second panel dataset (say, file2.gdt) to an open panel dataset (say, file1.gdt) is perhaps made "too" simple with gretl, potentially resulting in a rubbish combined dataset. The core problem seems to be that gretl (for appending) matches the internal numbering of panel units, even though this internal numbering isn't really meaningful. Let me try to explain: The attached "file1_test.gdt" holds unbalanced data for 4 panel units, while "file2_test.gdt" holds data for 5 panel units. The overall time dimension for both is the same. With the 1st dataset loaded, when I append the 2nd dataset (simple 'append', no 'join' or whatever), gretl does that without any complaints. But suppose that the 4 panel units in file1 do _not_ correspond to the first 4 units in file2. This is something that can easily happen in real life. So the append operation doesn't get the matching right. In addition, I tried a variation where the two panel data structures were defined properly with panel index variables that had no missings. Still, 'append' doesn't care about that, always just referring to the internal but separate unit index numbering. Yes, I'm aware that I can get the desired result using 'join', that's not the point. The point is about the following claim from the 'append' function reference: "In the case of adding series, compatibility requires either (a) that the number of observations for the new data equals that for the current data, or (b) that the new data carries clear observation information so that gretl can work out how to place the values." -- In my example, condition (a) is clearly not the case, and I'd argue that condition (b) is either violated as well, or that the panel-index-variable information is ignored by 'append'. Perhaps 'append' should refuse to proceed in my example case, or it should ask the user, or it should make use of the panel index variables, but arguably it should not use the arbitrary internal unit numbers. I hope that the problem description was clear enough! I'm curious what your reactions and answers will be, thanks sven