Group-Analytic Contexts, Issue 76, June 2017

90 Group-Analytic Society International - Contexts
What’s noise? It’s the variation. Signal is the size of the
difference from the null hypothesis. (Often called effect size). Applied
to your sample size in the above example, you believe that the twice
weekly therapy increases wellbeing rates from 50% to 70%. In this
example, the effect size is 20%. Put into the formula, you would need
a minimum of 250 clients to show that twice weekly therapy is more
effective than once weekly groups.
Let’s try another example: if you wanted to see whether twice weekly
psychotherapy improves wellbeing rates from 50% to 60%, your
effect size is 10%. If you conduct your study and use an alpha of 5%,
an expected rate of 60% in your experimental group and 50% in you
control group, you would need more than 1000 clients, according to
the above formula.
That’s all there is to it. So, the next time, you plan a study
which has a hypothesis, do a power calculation when planning it, so
you don’t miss any positive results.
Cheerio for now,
Susanne Vosmer
s.vosmer@gmail.com