Sample size calculation for linear mixed-effect models without data I would like to know how to do a sample size calculation without data/simulation for mixed-effect models.
To briefly explain my research, I want to investigate which method (A or B) is more effective for promoting students' participation in which the students are grouped for discussion.

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*2 between-subject condition: A or B

*1 dependent variable: participation

*1 random-effect variable: group

I will build two mixed-effect linear models and conduct 'ANOVA' to get the significance of the condition:

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*modelA = participation ~ condition + (1|group)

*modelB = participation ~ 1 + (1|group)

*anova(modelA, modelB)

Q1. Would it be fine to conduct sample size calculation using simple ANOVA-based power analysis (like G*Power)? In this case, the random-effect won't be considered, and was wondering if this is important. If it is important, how much difference in sample size would it make? If it is not important, why?
Q2. I know that there are some programs like simr that is for mixed-effect models, but they require pilot data or estimations for data parameters. I don't have those data and I only have 3 parameters with conventional values: d=0.5, a=0.05, power=0.8. Are there any method that I can get a sample size using these three parameters?
Thank you in advance!
 A: 
Q1. Would it be fine to conduct sample size calculation using simple ANOVA-based power analysis (like G*Power)? In this case, the random-effect won't be considered, and was wondering if this is important. If it is important, how much difference in sample size would it make? If it is not important, why?

No, it is not fine to ignore the random effects. The greater the intra-class correlation, the lower will be the effective sample size.

Q2. I know that there are some programs like simr that is for mixed-effect models, but they require pilot data or estimations for data parameters. I don't have those data and I only have 3 parameters with conventional values: d=0.5, a=0.05, power=0.8. Are there any method that I can get a sample size using these three parameters?

Well, you could do the simulations from scratch by yourself, but you are still going to need values for all relevant parameters. If you don't have them then you can use a range of estimates from other studies, or from expert domain knowledge.
