One difference is that as the experiments were run on psychology students, they know the population, those are their peers with whom they interact on a daily level and they should have an idea of how they compare with them.
> how would they know that how their classmates perform on an IQ test???
Are you serious? If you're interacting with your classmates, you definitely should have some idea on how their intellectual capabilities differ between each other and also with respect to you. In a small class doing lots of things together, someone might even literally count their "ranking" at some metric that highly correlates with IQ, estimating that Bob, Jane and Mary are above me and Dan and Juliet are below me, so I'm at 40th percentile.
It's not appropriate to treat these aspects as unknown things or unknowable things.
> how would they know that how their classmates perform on an IQ test???
Are you serious? If you're interacting with your classmates, you definitely should have some idea on how their intellectual capabilities differ between each other and also with respect to you. In a small class doing lots of things together, someone might even literally count their "ranking" at some metric that highly correlates with IQ, estimating that Bob, Jane and Mary are above me and Dan and Juliet are below me, so I'm at 40th percentile.
It's not appropriate to treat these aspects as unknown things or unknowable things.