In biostatistics, when comparing the means of two independent groups of subjects using an unpaired Student t test, the *effect size* is expressed as the ratio of Δ (*delta**,* the difference between the means of two groups) divided by σ (*sigma**,* the within-group standard deviation).

Each chart in the following figure shows overlapping bell curves that indicate the amount of separation between two groups, along with the effect size (Δ/σ) and the required number of analyzable subjects in each group*.* Pick the chart that looks like an important amount of separation between the two groups. For example, if the middle chart (corresponding to a between-group difference that’s three-fourths as large as the within-group standard deviation) looks like an important amount of separation, then you need about 29 analyzable subjects per group (for a total of 58 analyzable subjects).

For other Δ/σ values, use this rule of thumb to estimate sample size: You need about 16/(Δ/σ)^{2} analyzable subjects in each group.