Overconfidence
Also called: overconfidence bias, illusion of skill, better than average effect
Overconfidence is systematically overestimating your own knowledge, precision and skill. It's the best-documented bias in investing and the most expensive: research on brokerage accounts found that the most active traders underperformed the least active by several percentage points a year, with turnover the direct cause. Narrow confidence intervals on forecasts are its clearest measurable signature.
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