Clinical trial sample size requires defining: Alpha (α) — type I error rate (usually 0.05 = 5% false positive), Beta (β) — type II error rate (usually 0.20, giving 80% power), Effect size (Cohen's d) — practical importance of the difference, Test type (one-sided vs two-sided). For comparing proportions (e.g., treatment response rates), use the Fisher exact or chi-square formula. For continuous outcomes (e.g., blood pressure reduction), use the t-test formula. Our calculator supports both.