  Hedges' d, an existing unbiased effect size of the difference between means,
assumes the variance equality. However, the assumption of the variance equality
is fragile, and is often violated in practical applications. Here, we define e,
a new effect size of the difference between means, which does not assume the
variance equality. In addition, another novel statistic c is defined as an
effect size of the difference between a mean and a known constant. Hedges' g,
our c, and e correspond to Student's unpaired two-sample t test, Student's
one-sample t test, and Welch's t test, respectively. An R package is also
provided to compute these effect sizes with their variance and confidence
interval.
