On my first "working" day of retirement I have made a new statistics video - it is over six months since I last published a video on YouTube. This is one of a few statistics videos that I wanted to do to plug a few gaps in the series that I have already produced. Statistics videos are now my most popular, so I'm hoping that this one will prove popular too.
Cohen's d measures effect size. This tells us how meaningful a difference is when we find a significant difference as a result of a statistics test. Usually we report a difference as p < 0.05, but this does not tell us if the difference is small, medium, or large - so it is important to calculate the effect size.
Here's how it's done for a one sample z test (it can also be used for a one sample t test):