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

squared error

The square of one example's residual, $(y - \hat{y})^2$ ; never negative, and larger misses count more than proportionally more.

Examples

Residual 2 gives squared error 4; residual $-4$ gives squared error 16.