Sample size calculation

#1
Hi there,

Could someone please help me get this sample size calculation right?
I would like to calculate the sample size for a randomized controlled trial.

Based on a systematic review, the effect of the primary outcome (insomnia severity) is expected to be Hedges's g= 1.03.
The power should be 0.80, alpha is 0.05 (two-tailed).
I am expecting a drop-out rate of 25%.

I found this formula:
1623412931117.png


If I want to fill in this formula I am missing the standard deviation.
Could someone please advise me on what is the best way to perform this calculation?
I do have Stata 16 aswell where I see options to do this analysis. However, I am not sure which exact method to use.

Best regards,
Laura
 

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