Klayman, Soll, González-Vallejo and Barlas (1999) compared two ways of asking people how sure they were. When people chose between two answers, they showed little overall overconfidence. When they gave ranges at a stated level of confidence, the overconfidence was pronounced. A range you are 90% sure of should hold the true value about nine times in ten.
Klayman, Soll, González-Vallejo & Barlas (1999). Overconfidence: It Depends on How, What, and Whom You Ask. Organizational Behavior and Human Decision Processes.