An estimator of the effect size that uses only the standard deviation of
the second group.
https://en.wikipedia.org/wiki/Effect_size#Glass.27_.CE.94
μ₁ - μ₂
Δ = -------
s₂
where
μ₁ = mean of sample population 1
μ₂ = mean of sample population 2
s₂ = standard deviation of sample population 2