We provide an introduction into power analysis based on a two-sample problem with normally distributed errors. We consider the impact of balancing the size of treatment groups, and methods for increasing precision and power by blocking or subsampling that do not require increasing the sample size. We then discuss the determination of sample sizes to achieve a desired precision of a difference of means, and the problem of power analysis for testing such a difference based on the normal and the t-distribution. We also provide some practical shorthand formulas for quick approximate sample size calculations.