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

conditional expectation

The mean of a conditional distribution -- what Y averages to, given that X took a specific value.

$E(Y\mid X=x)=\sum_y y\,p_{Y\mid X}(y\mid x)$ .