Effect Size: Cohen’s d & Hedges’ g
Effect sizes in R with the effectsize package — Cohen’s d, the small-sample-corrected Hedges’ g, and how to interpret the magnitude.
A p-value says whether an effect exists; an effect size says how big it is.
Run the code
Everything in the video — edit it and press Run Code to run real R right here, or copy it:
Note
The first Run Code installs the packages this lesson needs (effectsize) into your browser session — give it 20–40 seconds.
Download this R script · ▶ Open the full playground
When to use it. Report an effect size alongside every p-value. Use Hedges’ g rather than Cohen’s d for small samples, and interpret with care (0.2 small, 0.5 medium, 0.8 large).
Try the Cohen’s d calculator · read Cohen’s d in R.