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The bias ensures that the intercept is greater than 0 and the random error ensures the slope is less than 1, and that’s all you need to reproduce the chart. You don’t need uniform belief in being better than average, you just need everyone to be the same amount of overconfident.
Notably you get a very similar chart if no one has any bias (everyone is correctly calibrated except for random error) or even if everyone has perfect belief about their own abilities but the test is noisy.
