The Dunning-Kruger effect is the phenomenon wherein people who have little knowledge think that they know more than others who have much more knowledge.[…] Across 4 studies, the authors found that participants scoring in the bottom quartile on tests of humor, grammar, and logic grossly overestimated their test performance and ability. Although test scores put them in the 12th percentile, they estimated themselves to be in the 62nd.

http://en.wikipedia.org/wiki/Dunning-Kruger_effect
http://en.wikipedia.org/wiki/Dunning-Kruger_effect
http://en.wikipedia.org/wiki/Dunning-Kruger_effect

Thanks jotajota!