Free public beta until December 4, 2026: every course is open, no card needed. After that, the first chapter of each course stays free and full access is a one-time $39 ($19 for up to 100 founding members chosen for helpful feedback). Your progress is kept. How the beta works

residual

Truth minus prediction for one example, $y - \hat{y}$ ; positive when the guess was too low, negative when too high.

Examples

A car sold for 24 and the rule guessed 22: the residual is $24 - 22 = 2$ .