Effect Size
Read
Coe (2002) It's the effect size, stupid: What effect size is and why it is important (read this first for a general introduction to effect size d)
Visualization of Cohen's d: Interpreting Cohen's d effect size: An interactive visualization
Lakens (2013) Calculating and reporting effect sizes to facilitate cumulative science: A practical primer for t-tests and ANOVAs (expands beyond effect size d)