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

covariance

A measure of how two random variables move together -- positive when they tend to rise and fall in tandem, negative when one runs high as the other runs low.

$\operatorname{Cov}(X,Y)=E(XY)-E(X)E(Y)$ .