What you are referring to is the Lake Wobegon or Illusionary Superiority effect. I cannot find the study you requested, but here are two similar studies. One found that 87% of MBA students thought they were above the median, while the other has that 67% of university faculty surveyed thought they were in the top 25%. Another thing you might want to look into is the Dunning-Kruger effect, where those who are incompetent don't know it. The study most likely references one of these three effects.