@defuneste That is actually helpful, because they specifically want to do interpolation on a panel design and then use design-based estimation on the interpolated data. I think I need some kind of evidence that you can't do this though—either an analogy, a quick demo with simulated data, or a some kind of published paper with those things in it. I don't think they're going to just take my word for it if I say "No, you can't do that"