I was reading some articles on interviewing and came across the Dunning–Kruger Effect. It describes how there is a cognitive bias in people of low ability where they mistakenly assess their ability as greater than it really is. In interviewing the concept points to how some unqualified people will over confidently present themselves. It also explains how those with average ability will appear less confident. This is a problem and one to be aware of because you may if not aware end up picking people based more on false confidence rather than actual ability.
The paper: