Effect size is how we measure improvement. An effect size of 1 means that the patient improved the equivalent of one standard deviation on the outcome measure. 

Effect sizes of 0.8 or larger are considered large, while effect sizes of 0.5 to 0.8 can be considered moderately large. Effect sizes of less than 0.3 are small and might well have occurred without any treatment at all.

Effect size is a simple way of quantifying the difference between two groups. It goes beyond tests of statistical significance alone. It pushed the question: 'Does it work or not?' to the far more sophisticated, 'How well does it work in a range of contexts?' 


The American Psychological Association has been officially encouraging authors to report effect sizes since 1994 - but with limited success."

-Robert Coe, Address to the British Educational Research Association.


The severity adjusted effect size is calculated because…

-severity at intake 


-the session number of the first assessment (i.e. if a client is mid-case)

need to be taken into account.

Q: Does filling in the diagnosis details on the Client Registration form change the way effect size is calculated?

A: Yes, effect size does take into account the diagnosis(es) you provide on the client Registration form. In reality, diagnosis accounts for little of the variance in outcome, with the exception of psychotic disorders. In our predictive modeling (multivariate general linear modeling) the diagnosis cluster accounts for less than 2% of the variance in outcome.