you just assumed a random sampling. these sorts of studies are literally the farthest you can get from random sampling- a bunch of uni students at a single uni in a single region.

Many of us went through study size calculations when we were first learning how to run clinical trials (well, probalby not many, but everybody in my cohort did). The general conclusion among quantitative scientists is that social science like this is worse than useless because of how much time you have to spend explaining to people that the world is not a simple uniform random sampling.