Oddly Wired
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judgment

Your certainty in your beliefs outweighs their actual accuracy.

Research by Fischhoff, Slovic, and Lichtenstein (1977) demonstrates that individuals consistently demonstrate overconfidence, often failing to reach 100% accuracy even when they express complete certainty.

The accuracy rate for claims of 100% certainty varies by task and is not a fixed universal value of 80%.

Being sure of yourself is not the same as being right.

Where this comes from

Study
Fischhoff, Slovic, and Lichtenstein, 1977
What the check found
Fischhoff, Slovic, and Lichtenstein (1977) did demonstrate that individuals are frequently overconfident when expressing extreme certainty, but 80% is an approximation of task performance rather than a rigid constant.
Confidence
Solid, with limits

Related

One of these every day, and every one names the study it came from. When a finding is contested, it says so.