Unpaired t-test
Find sample size: If you can estimate group means and standard deviation, use this form to find the number of subjects you need. Group 1 mean: Group 2 mean: or, enter difference between means: Standard deviation: Click here for sample size: You will need subjects in Group 1 You will need subjects in Group 2	Find effect size: If you know the number of subjects and the standard deviation of your measurement, use this form to see how small a difference you can detect. N for Group 1: Standard deviation: Click here for effect size: You can show a difference of size
For different power or significance level, change the fields below: Alpha: Prob(reject H0 when H0 is true) 0.05 0.01 0.005 0.001 0.0005 0.0001 Power: Prob(reject H0 when H1 is true) 0.80 0.90
For unequal sample sizes, change the ratio below: Group2/Group1:
Choose another study design v. 03/03 Contact: sfh2@columbia.edu Source: G.W. Snedecor & W.G. Cochran Statistical Methods (7th ed., p. 104)