I have just had my viva and my sample size calculation was criticised as it was based on r2. I was told to base the sample size on the minimal magnitude of association. My outcome variable is HbA1c, a marker of blood glucose. The minimal magnitude of association is 6 mmol/mol. I am reporting unstandardised b. I have used mixed effects multi level models and have 15 covariates.
I am ok with accounting for clustering once i have a sample size but was wondering how i calculate the sample size? I think that i am getting confused with effect sizes.....Where would i enter my value thats considered of clinical importance? Or does this translate to the effect size? I am using GPower.
Any help would be greatly appreciated. Thank you!