If you have a selection of ‘magic’ sugar pills, and you want to test them for being magic vs placebo effect, you do a study comparing their efficacy to that of ‘non-magic’ sugar pills.
If they are magic, then you aren’t comparing identical things, because only some of them have the ‘magic’ property
If you have a selection of ‘magic’ sugar pills, and you want to test them for being magic vs placebo effect, you do a study comparing their efficacy to that of ‘non-magic’ sugar pills.
If they are magic, then you aren’t comparing identical things, because only some of them have the ‘magic’ property