In this workshop we will discuss different approaches to justifying the sample size of your study, and focus on statistical power analysis. We will discuss the basic concepts of statistical power through simulations, and through group discussions and hands-on assignments think about the steps required to decide upon the sample size for future studies. We will consider ways to deal with the uncertainty in the expected effect size, ask ourselves whether we can trust effect sizes from the published literature, and if not, how to deal with bias, how to design studies when planning for the absence of an effect, and compare statistical power in Bayesian and Frequentist sequential analysis. At the end of this workshop, you will be able to design efficient studies and justify the sample size in articles you will submit for publication.