27.2 Arguments to pwr.t.test:

• n: Number of observations (per sample)
• d: Effect size (Cohen’s d) - difference between the means divided by the pooled standard deviation
• sig.level: Significance level (Type I error probability)
• power: Power of test (1 minus Type II error probability)
• type: Type of t test : one- two- or paired-samples
• alternative: a character string specifying the alternative hypothesis,
  • must be one of “two.sided” (default), “greater” or “less”

\(d\) is the effect size (diff in means divided by mean within group std dev) \[d = \frac{\text{difference between a pair of group means}}{\text{averaged group standard deviation}}\]