The Dunning-Kruger effect.
Justin Kruger & David Dunning.
Unskilled and unaware of It: how difficulties in recognizing one's own incompetence lead to inflated self-assessments.
Journal of personality and social psychology
1999, vol.77, no.6, pp1121-1134
©1999 by American Psychological Association, Inc.
For those unfamiliar with percentile rankings, briefly....
Say you get 95% of questions right in a test — sounds good. But if everyone else does, then your performance is actually average. Percentiles are a method of marking, which ranks your score COMPARATIVE to other scores within a population.
If you rank at the 10th percentile, 10 percent of all scores, in the population tested, fall BELOW you. If you rank at the 90th percentile, 90 percent of all test results fall BELOW you — you've made it to the top 10. 50% is dead on average.
Bizarrely, in a tendency which has been termed the 'above-average effect', it seems most people — no matter what their competence/ability — perceive themselves as above average, hovering around the 60th percentile.