I studied this many years ago, but you have formulas for survey sizes based on confidence levels and, if I recall correctly, the number of variables you want to

I studied this many years ago, but you have formulas for survey sizes based on confidence levels and, if I recall correctly, the number of variables you want to study. People publishing these studies should know and use these formulas, but I imagine there's a lot of pressure to publish high impact/visibility stuff so they just go for the cheapest and fastest (aka wrong) approaches some times.