As a chartered clinical psychologist within the British Psychology Society, I get sent a magazine called “The Psychologist” every month. In the August 2013 issue there was an article called “Why are effect sizes still neglected?” by Peter Morris and Catherine Fritz. The gist of the article is that when psychologist evaluate treatments using hypothesis testing, the results are often misleading. In particular, if you have large enough sample size you will almost always get a statistically significant result, even for minor and inconsequential effects. Consequently, it is important to always report an effect size when publishing results in an academic journal.

A common way to establish the effect size is to compare the symptoms for patients before and after a treatment. In the following video I give a tutorial on how to use data stored in an Excel spreadsheet with the AI-Therapy effect size calculator (it’s best to view full screen at 720p HD):


Fjola  Helgadottir, PhD, MClinPsych, is a clinical psychologist, a senior research clinician at the University of Oxford, and is a co-creator of, an online CBT treatment program for overcoming social anxiety