Determining Sample Size (for a Test of the Difference Between Two Means)

When conducting a test for the difference between two means, given the effect size (Δ = (μ1 - μ2)/σ—how many times the standard deviation is the difference between the means?) and the significance level, this formula calculates the required sample size n (where n = n1 = n2; the meaning of each sample size).　Null hypothesis: μ1 = μ2; for the alternative hypothesis, you can choose from three options: μ1 ≠ μ2, μ1 > μ2, or μ1 < μ2.