[kr7]

The choice does not have to be between "do a bad study" or "do no study at all".

A proper study would be permanent and self-contained. Participants would need to give a portion of outside income to the program to simulate higher taxes and/or inflation.

[cwiggs]

You mention make the UBI be permanent so, how long would you expect to wait until the results would be worth looking at, an entire generation?

[notahacker]

Results can start to be worthwhile early on. But the life-altering decisions people make over a two year period when guaranteed a subsidy that runs out at the end of those two years are not particularly likely to be the same as the life altering decisions people make over a two year period when guaranteed that subsidy for life.

(If you wanted to really understand all the dynamics, you'd probably want to wait more than a generation to understand how subsidy recipients' decisions to raise children are affected and whether those children make systematically different life choices.)