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

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Margin of Error Sprint

z*-times-standard-error, three ways -- a plain margin, a full confidence-interval bound, and a proportion's own square-root formula.

⏱️ 90s

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