What actually makes a participant "qualified" for a study

It’s tempting to think of eligibility as a filter: does this person meet the bar, yes or no. In practice it’s closer to a fit function -- how well does this person’s profile match what this specific study needs to learn something real.

Screen too loosely and you get data that doesn’t answer your research question, because the people responding weren’t the people you needed to hear from. Screen too tightly and you either can’t fill the study or you end up with a sample so narrow it doesn’t generalize to anything.

The requirements that matter are the ones tied directly to what the study is trying to determine -- not a generic demographic checklist applied out of habit. A usability study on a new feature needs people who’d plausibly use that feature, not a broad cross-section of the population.

This is also why eligibility should be treated as part of the study design, not an afterthought bolted on after the questions are written. The requirements you set determine whether the answers you get are answers to the question you actually asked.