I am conducting a one-sided, independent two-sample t-test for a gender study and I can guesstimate my effect size from previous studies. I want 95% significance, 80% power. I am looking to calculate appropriate sample size and realise I need the standard deviation to do this.
Is there any way of approximating the standard deviation from the effect size? My lecturer said doubling the effect size provides a rough approximation of an appropriate standard deviation. Is this correct? Why is this the case??
Thanks in advance.
Note: effect size is around +16 percentage points (control group mean estimation: 34%, treatment group mean estimation: 50%).