Sample size for comparing two proportions

Parameters

Significance threeshold (α)	0.025	0.05	0.10	0.20
	One-sided		Two-sided
Power (1 - β)	0.975	0.95	0.90	0.80
Outcome rate in the control group
You may enter multiple proportions or percentages, delimited by commas or spaces (0.1, 10, 10%). If uncertain, leave blank or enter 0.50.
Minimal clinically important difference (δ)
Outcome rate in the intervention group minus outcome rate in the control group.

References

Casagrande JT, Pike MC, Smith PG. An improved approximate formula for calculating sample sizes for comparing two binomial distributions. Biometrics 1978;34:483-6.
Fleiss JL, Tytun A, Ury HK. A simple approximation for calculating sample sizes for comparing independent proportions. Biometrics 1980;36:343-6.

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