I think you’re confused here. The $50 group (Group C) is the control group. That sum was chosen because it was assumed it wouldn’t actually meaningfully change

I think you’re confused here. The $50 group (Group C) is the control group. That sum was chosen because it was assumed it wouldn’t actually meaningfully change their lives, and therefore we can use their response as a baseline for how to judge Group A and Group B. This would imply that universal basic income makes 20% of people feel like they’re doing better (placebo effect), and 6% of people actually do better.