Hello, all! I am new here and would love help for this question.

I am running a waitlist control study in which the Independent Variable is receiving a treatment manual for trauma. Group 1 receives 8 weeks of the treatment first, followed by 8 weeks where Group 2 receives the treatment. My advisor suggests that we run a multiple regression on the data at the end (quantitative measures of spiritual wellness and spiritual trauma). I am using G*Power to calculate my needed sample size and I am unsure of how to go about finding out my sample size. Here are my requirements for power:

Effect size f=0.25,

alpha err prob=0.05,

Power (1-Beta err prob)=0.8,

Number of groups=2(I believe, based on my design listed above)

Thank you very much!